diff --git a/Cargo.toml b/Cargo.toml
index 82bd96bb..ea085632 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -29,6 +29,16 @@ japanese = [
     "erg_parser/japanese",
     "erg_compiler/japanese",
 ]
+simplified_chinese = [
+    "erg_common/simplified_chinese",
+    "erg_parser/simplified_chinese",
+    "erg_compiler/simplified_chinese",
+]
+traditional_chinese = [
+    "erg_common/traditional_chinese",
+    "erg_parser/traditional_chinese",
+    "erg_compiler/traditional_chinese",
+]
 
 [dependencies]
 erg_common = { version = "0.2.5", path = "./compiler/erg_common" }
diff --git a/assets/screenshot_i18n_messages.png b/assets/screenshot_i18n_messages.png
new file mode 100644
index 00000000..1936492b
Binary files /dev/null and b/assets/screenshot_i18n_messages.png differ
diff --git a/compiler/erg_common/Cargo.toml b/compiler/erg_common/Cargo.toml
index 418aca0c..45d3dcb1 100644
--- a/compiler/erg_common/Cargo.toml
+++ b/compiler/erg_common/Cargo.toml
@@ -14,6 +14,8 @@ homepage = "https://erg-lang.github.io/"
 [features]
 debug = []
 japanese = []
+simplified_chinese = []
+traditional_chinese = []
 
 [dependencies]
 atty = "0.2.14"
diff --git a/compiler/erg_common/deserialize.rs b/compiler/erg_common/deserialize.rs
index cb29b67f..e0f8212e 100644
--- a/compiler/erg_common/deserialize.rs
+++ b/compiler/erg_common/deserialize.rs
@@ -58,8 +58,8 @@ impl DeserializeError {
             0,
             fn_name!(),
             switch_lang!(
-                "the loaded .pyc file is broken",
-                "読み込んだ.pycファイルは破損しています"
+                "japanese" => "読み込んだ.pycファイルは破損しています",
+                "english" => "the loaded .pyc file is broken",
             ),
         )
     }
@@ -69,14 +69,14 @@ impl DeserializeError {
             0,
             fn_name!(),
             switch_lang!(
-                format!(
+                "japanese" => format!(
+                    "{}型オブジェクトを予期しましたが、 読み込んだオブジェクトは{}型です",
+                    expect, found
+                ),
+                "english" => format!(
                     "expect a {} object, but the deserialized object is {}",
                     expect, found
                 ),
-                format!(
-                    "{}型オブジェクトを予期しましたが、 読み込んだオブジェクトは{}型です",
-                    expect, found
-                )
             ),
         )
     }
@@ -228,8 +228,8 @@ impl Deserializer {
                 0,
                 fn_name!(),
                 switch_lang!(
-                    format!("cannot deserialize this object: {}", other),
-                    format!("このオブジェクトは復元できません: {}", other)
+                    "japanese" => format!("このオブジェクトは復元できません: {}", other),
+                    "english" => format!("cannot deserialize this object: {}", other),
                 ),
             )),
         }
@@ -300,7 +300,10 @@ impl Deserializer {
             return Err(DeserializeError::new(
                 0,
                 fn_name!(),
-                switch_lang!("failed to load bytes", "バイト列の読み込みに失敗しました"),
+                switch_lang!(
+                    "japanese" => "バイト列の読み込みに失敗しました",
+                    "english" => "failed to load bytes",
+                ),
             ));
         }
         let len = Self::deserialize_u32(v);
diff --git a/compiler/erg_common/error.rs b/compiler/erg_common/error.rs
index d5cc76bb..af737181 100644
--- a/compiler/erg_common/error.rs
+++ b/compiler/erg_common/error.rs
@@ -321,10 +321,16 @@ impl ErrorCore {
     }
 
     pub fn bug(errno: usize, loc: Location, fn_name: &str, line: u32) -> Self {
-        Self::new(errno, CompilerSystemError, loc, switch_lang!(
-            format!("this is a bug of Erg, please report it to https://github.com/...\ncaused from: {fn_name}:{line}"),
-            format!("これはErgのバグです、開発者に報告して下さい (https://github.com/...)\n{fn_name}:{line}より発生")
-        ), None)
+        Self::new(
+            errno,
+            CompilerSystemError,
+            loc,
+            switch_lang!(
+                "japanese" => format!("これはErgのバグです、開発者に報告して下さい (https://github.com/...)\n{fn_name}:{line}より発生"),
+                "english" => format!("this is a bug of Erg, please report it to https://github.com/...\ncaused from: {fn_name}:{line}"),
+            ),
+            None,
+        )
     }
 }
 
diff --git a/compiler/erg_common/macros.rs b/compiler/erg_common/macros.rs
index bb22604b..592e3949 100644
--- a/compiler/erg_common/macros.rs
+++ b/compiler/erg_common/macros.rs
@@ -58,11 +58,17 @@ macro_rules! impl_display_for_enum_with_variant {
 /// マクロはパラメータを展開しないので、format!のロスがなくなる
 #[macro_export]
 macro_rules! switch_lang {
-    ($en: expr, $jp: expr $(,)*) => {{
-        if cfg!(feature = "japanese") {
-            $jp
+    (
+        $should_english: literal => $msg: expr,
+    ) => {{ $msg }};
+    (
+        $lang_name: literal => $msg: expr,
+        $($rest_lang_name: literal => $rest_msg: expr,)+
+    ) => {{
+        if cfg!(feature = $lang_name) {
+            $msg
         } else {
-            $en
+            switch_lang!($($rest_lang_name => $rest_msg,)+)
         }
     }};
 }
diff --git a/compiler/erg_common/serialize.rs b/compiler/erg_common/serialize.rs
index a4d2717b..af1f517f 100644
--- a/compiler/erg_common/serialize.rs
+++ b/compiler/erg_common/serialize.rs
@@ -49,14 +49,14 @@ pub enum DataTypePrefix {
     StopIter = b'S',   // 0x53
     Ref = b'r',
     /* unsized objects (ref counted) */
-    Long = b'l',       // 0x6C + len: u32 + payload: 2*len+3byte (~ -2^31-1 && 2^31 ~)
-    Str = b's',        // 0x73 + len: u32 + payload
-    ShortAscii = b'z', // 0x7A + len: u8 + payload
-    ShortAsciiInterned = b'Z', //  0x5A + len: u8 + payload
-    Unicode = b'u',    // 0x75 + len: u32 + payload
+    Long = b'l',       // 0x6C + len:  u32 + payload: 2*len+3byte (~ -2^31-1 && 2^31 ~)
+    Str = b's',        // 0x73 + len:  u32 + payload
+    ShortAscii = b'z', // 0x7A + len:  u8 + payload
+    ShortAsciiInterned = b'Z', //  0x5A + len:  u8 + payload
+    Unicode = b'u',    // 0x75 + len:  u32 + payload
     Interned = b't',   // 0x74 + len + payload
-    SmallTuple = b')', // 0x29 + len: u8 + payload
-    Tuple = b'(',      // 0x28 + len: u32 + payload
+    SmallTuple = b')', // 0x29 + len:  u8 + payload
+    Tuple = b'(',      // 0x28 + len:  u32 + payload
     Code = b'c',       // 0x63
     /* Erg specific prefix */
     Builtin = b'b', // 0x62 + str
diff --git a/compiler/erg_common/value.rs b/compiler/erg_common/value.rs
index bcd348d5..d4c7ac2a 100644
--- a/compiler/erg_common/value.rs
+++ b/compiler/erg_common/value.rs
@@ -326,8 +326,8 @@ impl ValueObj {
                 panic!(
                     "{}",
                     switch_lang!(
-                        format!("this object cannot be serialized: {other}"),
-                        format!("このオブジェクトはシリアライズできません: {other}")
+                        "japanese" => format!("このオブジェクトはシリアライズできません: {other}"),
+                        "english" => format!("this object cannot be serialized: {other}"),
                     )
                 )
             }
diff --git a/compiler/erg_compiler/Cargo.toml b/compiler/erg_compiler/Cargo.toml
index 9a141d04..7de1b1f3 100644
--- a/compiler/erg_compiler/Cargo.toml
+++ b/compiler/erg_compiler/Cargo.toml
@@ -13,6 +13,8 @@ homepage = "https://erg-lang.github.io/"
 # when "debug" feature is turned on, that of parser will also be turned on.
 debug = [ "erg_common/debug", "erg_parser/debug" ]
 japanese = [ "erg_common/japanese", "erg_parser/japanese" ]
+simplified_chinese = [ "erg_common/simplified_chinese", "erg_parser/simplified_chinese" ]
+traditional_chinese = [ "erg_common/traditional_chinese", "erg_parser/traditional_chinese" ]
 
 [dependencies]
 erg_common = { version = "0.2.5", path = "../erg_common" }
diff --git a/compiler/erg_compiler/error.rs b/compiler/erg_compiler/error.rs
index c2f45080..cf221c0e 100644
--- a/compiler/erg_compiler/error.rs
+++ b/compiler/erg_compiler/error.rs
@@ -138,10 +138,20 @@ impl CompileError {
         fn_name: &str,
         line: u32,
     ) -> Self {
-        Self::new(ErrorCore::new(errno, CompilerSystemError, loc, switch_lang!(
-            format!("this is a bug of the Erg compiler, please report it to https://github.com/...\ncaused from: {fn_name}:{line}"),
-            format!("これはErg compilerのバグです、開発者に報告して下さい (https://github.com/...)\n{fn_name}:{line}より発生")
-        ), None), input, "".into())
+        Self::new(
+            ErrorCore::new(
+                errno,
+                CompilerSystemError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("これはErg compilerのバグです、開発者に報告して下さい (https://github.com/...)\n{fn_name}:{line}より発生"),
+                    "english" => format!("this is a bug of the Erg compiler, please report it to https://github.com/...\ncaused from: {fn_name}:{line}"),
+                ),
+                None,
+            ),
+            input,
+            "".into(),
+        )
     }
 
     pub fn stack_bug(
@@ -151,14 +161,24 @@ impl CompileError {
         block_id: usize,
         fn_name: &str,
     ) -> Self {
-        Self::new(ErrorCore::new(0, CompilerSystemError, loc, switch_lang!(
-            format!("the number of elements in the stack is invalid (num of elems: {stack_len}, block id: {block_id})\n\
-                    this is a bug of the Erg compiler, please report it (https://github.com/...)\n\
-                    caused from: {fn_name}"),
-            format!("スタックの要素数が異常です (要素数: {stack_len}, ブロックID: {block_id})\n\
-                    これはコンパイラのバグです、開発者に報告して下さい (https://github.com/...)\n\
-                    {fn_name}より発生")
-        ), None), input, "".into())
+        Self::new(
+            ErrorCore::new(
+                0,
+                CompilerSystemError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("スタックの要素数が異常です (要素数: {stack_len}, ブロックID: {block_id})\n\
+                            これはコンパイラのバグです、開発者に報告して下さい (https://github.com/...)\n\
+                            {fn_name}より発生"),
+                    "english" => format!("the number of elements in the stack is invalid (num of elems: {stack_len}, block id: {block_id})\n\
+                            this is a bug of the Erg compiler, please report it (https://github.com/...)\n\
+                            caused from: {fn_name}"),
+                ),
+                None,
+            ),
+            input,
+            "".into(),
+        )
     }
 
     pub fn feature_error(input: Input, loc: Location, name: &str, caused_by: Str) -> Self {
@@ -168,8 +188,9 @@ impl CompileError {
                 FeatureError,
                 loc,
                 switch_lang!(
-                    format!("this feature({name}) is not implemented yet"),
-                    format!("この機能({name})はまだ正式に提供されていません")
+                    "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+                    "simplified_chinese" => format!("该功能({name})还没有正式提供"),
+                    "english" => format!("this feature({name}) is not implemented yet"),
                 ),
                 None,
             ),
@@ -195,10 +216,19 @@ impl TyCheckError {
     }
 
     pub fn checker_bug(errno: usize, loc: Location, fn_name: &str, line: u32) -> Self {
-        Self::new(ErrorCore::new(errno, CompilerSystemError, loc, switch_lang!(
-            format!("this is a bug of the Erg compiler, please report it to https://github.com/...\ncaused from: {fn_name}:{line}"),
-            format!("これはErg compilerのバグです、開発者に報告して下さい (https://github.com/...)\n{fn_name}:{line}より発生")
-        ), None), "".into())
+        Self::new(
+            ErrorCore::new(
+                errno,
+                CompilerSystemError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("これはErg compilerのバグです、開発者に報告して下さい (https://github.com/...)\n{fn_name}:{line}より発生"),
+                    "english" => format!("this is a bug of the Erg compiler, please report it to https://github.com/...\ncaused from: {fn_name}:{line}"),
+                ),
+                None,
+            ),
+            "".into(),
+        )
     }
 
     pub fn feature_error(loc: Location, name: &str, caused_by: Str) -> Self {
@@ -208,8 +238,8 @@ impl TyCheckError {
                 FeatureError,
                 loc,
                 switch_lang!(
-                    format!("this feature({name}) is not implemented yet"),
-                    format!("この機能({name})はまだ正式に提供されていません")
+                    "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+                    "english" => format!("this feature({name}) is not implemented yet"),
                 ),
                 None,
             ),
@@ -238,8 +268,8 @@ impl TyCheckError {
                 NameError,
                 loc,
                 switch_lang!(
-                    format!("{name} is already declared"),
-                    format!("{name}は既に宣言されています")
+                    "japanese" => format!("{name}は既に宣言されています"),
+                    "english" => format!("{name} is already declared"),
                 ),
                 Option::<Str>::None,
             ),
@@ -255,12 +285,18 @@ impl TyCheckError {
         found_t: &Type,
     ) -> Self {
         let name = readable_name(name);
-        Self::new(ErrorCore::new(0, TypeError, loc,
-            switch_lang!(
-                format!("{name} was declared as {GREEN}{spec_t}{RESET}, but an {RED}{found_t}{RESET} object is assigned"),
-                format!("{name}は{GREEN}{spec_t}{RESET}型として宣言されましたが、{RED}{found_t}{RESET}型のオブジェクトが代入されています")
-            ), Option::<Str>::None),
-            caused_by
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("{name}は{GREEN}{spec_t}{RESET}型として宣言されましたが、{RED}{found_t}{RESET}型のオブジェクトが代入されています"),
+                    "english" => format!("{name} was declared as {GREEN}{spec_t}{RESET}, but an {RED}{found_t}{RESET} object is assigned"),
+                ),
+                Option::<Str>::None,
+            ),
+            caused_by,
         )
     }
 
@@ -272,8 +308,8 @@ impl TyCheckError {
                 TypeError,
                 loc,
                 switch_lang!(
-                    format!("the type of {name} is not specified"),
-                    format!("{name}の型が指定されていません")
+                    "japanese" => format!("{name}の型が指定されていません"),
+                    "english" => format!("the type of {name} is not specified"),
                 ),
                 None,
             ),
@@ -291,8 +327,8 @@ impl TyCheckError {
         let hint = similar_name.map(|n| {
             let n = readable_name(n);
             switch_lang!(
-                format!("exists a similar name variable: {n}"),
-                format!("似た名前の変数があります: {n}")
+                "japanese" => format!("似た名前の変数があります: {n}"),
+                "english" => format!("exists a similar name variable: {n}"),
             )
             .into()
         });
@@ -302,8 +338,8 @@ impl TyCheckError {
                 NameError,
                 loc,
                 switch_lang!(
-                    format!("{RED}{name}{RESET} is not defined"),
-                    format!("{RED}{name}{RESET}という変数は定義されていません")
+                    "japanese" => format!("{RED}{name}{RESET}という変数は定義されていません"),
+                    "english" => format!("{RED}{name}{RESET} is not defined"),
                 ),
                 hint,
             ),
@@ -321,8 +357,8 @@ impl TyCheckError {
         let hint = similar_name.map(|n| {
             let n = readable_name(n);
             switch_lang!(
-                format!("has a similar name attribute: {n}"),
-                format!("似た名前の属性があります: {n}")
+                "japanese" => format!("似た名前の属性があります: {n}"),
+                "english" => format!("has a similar name attribute: {n}"),
             )
             .into()
         });
@@ -332,8 +368,8 @@ impl TyCheckError {
                 AttributeError,
                 loc,
                 switch_lang!(
-                    format!("{obj_t} object has no attribute {RED}{name}{RESET}"),
-                    format!("{obj_t}型オブジェクトに{RED}{name}{RESET}という属性はありません")
+                    "japanese" => format!("{obj_t}型オブジェクトに{RED}{name}{RESET}という属性はありません"),
+                    "english" => format!("{obj_t} object has no attribute {RED}{name}{RESET}"),
                 ),
                 hint,
             ),
@@ -353,12 +389,12 @@ impl TyCheckError {
                 NotImplementedError,
                 callee.loc(),
                 switch_lang!(
-                    format!(
+                    "japanese" => format!(
+                        "{callee}は{param_ts}を引数に取る呼び出し可能オブジェクトではありません"
+                    ),
+                    "english" => format!(
                         "{callee} is not a Callable object that takes {param_ts} as an argument"
                     ),
-                    format!(
-                        "{callee}は{param_ts}を引数に取る呼び出し可能オブジェクトではありません"
-                    )
                 ),
                 None,
             ),
@@ -373,10 +409,19 @@ impl TyCheckError {
         expect: &Type,
         found: &Type,
     ) -> Self {
-        Self::new(ErrorCore::new(0, TypeError, loc, switch_lang!(
-            format!("the type of {name} is mismatched:\nexpected:  {GREEN}{expect}{RESET}\nbut found: {RED}{found}{RESET}"),
-            format!("{name}の型が違います。\n予期した型: {GREEN}{expect}{RESET}\n与えられた型: {RED}{found}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("{name}の型が違います。\n予期した型: {GREEN}{expect}{RESET}\n与えられた型: {RED}{found}{RESET}"),
+                    "english" => format!("the type of {name} is mismatched:\nexpected:  {GREEN}{expect}{RESET}\nbut found: {RED}{found}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn return_type_error(
@@ -386,10 +431,19 @@ impl TyCheckError {
         expect: &Type,
         found: &Type,
     ) -> Self {
-        Self::new(ErrorCore::new(0, TypeError, loc, switch_lang!(
-            format!("the return type of {name} is mismatched:\nexpected:  {GREEN}{expect}{RESET}\nbut found: {RED}{found}{RESET}"),
-            format!("{name}の戻り値の型が違います。\n予期した型: {GREEN}{expect}{RESET}\n与えられた型: {RED}{found}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("{name}の戻り値の型が違います。\n予期した型: {GREEN}{expect}{RESET}\n与えられた型: {RED}{found}{RESET}"),
+                    "english" => format!("the return type of {name} is mismatched:\nexpected:  {GREEN}{expect}{RESET}\nbut found: {RED}{found}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn uninitialized_error(loc: Location, caused_by: Str, name: &str, t: &Type) -> Self {
@@ -399,8 +453,8 @@ impl TyCheckError {
                 NameError,
                 loc,
                 switch_lang!(
-                    format!("{name}: {t} is not initialized"),
-                    format!("{name}: {t}は初期化されていません")
+                    "japanese" => format!("{name}: {t}は初期化されていません"),
+                    "english" => format!("{name}: {t} is not initialized"),
                 ),
                 None,
             ),
@@ -409,10 +463,19 @@ impl TyCheckError {
     }
 
     pub fn argument_error(loc: Location, caused_by: Str, expect: usize, found: usize) -> Self {
-        Self::new(ErrorCore::new(0, TypeError, loc, switch_lang!(
-            format!("the number of positional arguments is mismatched:\nexpected:  {GREEN}{expect}{RESET}\nbut found: {RED}{found}{RESET}"),
-            format!("ポジショナル引数の数が違います。\n予期した個数: {GREEN}{expect}{RESET}\n与えられた個数: {RED}{found}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("ポジショナル引数の数が違います。\n予期した個数: {GREEN}{expect}{RESET}\n与えられた個数: {RED}{found}{RESET}"),
+                    "english" => format!("the number of positional arguments is mismatched:\nexpected:  {GREEN}{expect}{RESET}\nbut found: {RED}{found}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn match_error(loc: Location, caused_by: Str, expr_t: &Type) -> Self {
@@ -422,8 +485,8 @@ impl TyCheckError {
                 TypeError,
                 loc,
                 switch_lang!(
-                    format!("not all patterns of type {expr_t} are covered"),
-                    format!("{expr_t}型の全パターンを網羅していません")
+                    "japanese" => format!("{expr_t}型の全パターンを網羅していません"),
+                    "english" => format!("not all patterns of type {expr_t} are covered"),
                 ),
                 None,
             ),
@@ -438,8 +501,8 @@ impl TyCheckError {
                 TypeError,
                 loc,
                 switch_lang!(
-                    format!("failed to infer the type of {expr}"),
-                    format!("{expr}の型が推論できません")
+                    "japanese" => format!("{expr}の型が推論できません"),
+                    "english" => format!("failed to infer the type of {expr}"),
                 ),
                 None,
             ),
@@ -463,8 +526,8 @@ impl TyCheckError {
                 AssignError,
                 loc,
                 switch_lang!(
-                    format!("cannot assign twice to the immutable variable {name}"),
-                    format!("定数{name}には再代入できません")
+                    "japanese" => format!("定数{name}には再代入できません"),
+                    "english" => format!("cannot assign twice to the immutable variable {name}"),
                 ),
                 None,
             ),
@@ -487,18 +550,18 @@ impl TyCheckError {
                 TypeError,
                 loc,
                 switch_lang!(
-                    format!(
+                    "japanese" => format!(
+                        "{name}に渡された引数の数が多すぎます。
+必要な引数の合計数: {GREEN}{params_len}{RESET}個
+渡された引数の数:   {RED}{pos_args_len}{RESET}個
+キーワード引数の数: {RED}{kw_args_len}{RESET}個"
+                    ),
+                    "english" => format!(
                         "too many arguments for {name}:
 total expected params:  {GREEN}{params_len}{RESET}
 passed positional args: {RED}{pos_args_len}{RESET}
 passed keyword args:    {RED}{kw_args_len}{RESET}"
                     ),
-                    format!(
-                        "{name}に渡された引数の数が多すぎます。
-必要な引数の合計数: {GREEN}{params_len}{RESET}個
-渡された引数の数:   {RED}{pos_args_len}{RESET}個
-キーワード引数の数: {RED}{kw_args_len}{RESET}個"
-                    )
                 ),
                 None,
             ),
@@ -519,8 +582,8 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
                 TypeError,
                 loc,
                 switch_lang!(
-                    format!("{name}'s argument {RED}{arg_name}{RESET} is passed multiple times"),
-                    format!("{name}の引数{RED}{arg_name}{RESET}が複数回渡されています")
+                    "japanese" => format!("{name}の引数{RED}{arg_name}{RESET}が複数回渡されています"),
+                    "english" => format!("{name}'s argument {RED}{arg_name}{RESET} is passed multiple times"),
                 ),
                 None,
             ),
@@ -541,9 +604,9 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
                 TypeError,
                 loc,
                 switch_lang!(
-            format!("{name} got unexpected keyword argument {RED}{param_name}{RESET}"),
-            format!("{name}には予期しないキーワード引数{RED}{param_name}{RESET}が渡されています")
-        ),
+                    "japanese" => format!("{name}には予期しないキーワード引数{RED}{param_name}{RESET}が渡されています"),
+                    "english" => format!("{name} got unexpected keyword argument {RED}{param_name}{RESET}"),
+                ),
                 None,
             ),
             caused_by,
@@ -558,8 +621,8 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
                 UnusedWarning,
                 loc,
                 switch_lang!(
-                    format!("{YELLOW}{name}{RESET} is not used"),
-                    format!("{YELLOW}{name}{RESET}は使用されていません")
+                    "japanese" => format!("{YELLOW}{name}{RESET}は使用されていません"),
+                    "english" => format!("{YELLOW}{name}{RESET} is not used"),
                 ),
                 None,
             ),
@@ -580,10 +643,19 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
             (None, Some(r)) => r,
             (None, None) => Location::Unknown,
         };
-        Self::new(ErrorCore::new(0, TypeError, loc, switch_lang!(
-            format!("unification failed:\nlhs: {YELLOW}{lhs_t}{RESET}\nrhs: {YELLOW}{rhs_t}{RESET}"),
-            format!("型の単一化に失敗しました:\n左辺: {YELLOW}{lhs_t}{RESET}\n右辺: {YELLOW}{rhs_t}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("型の単一化に失敗しました:\n左辺: {YELLOW}{lhs_t}{RESET}\n右辺: {YELLOW}{rhs_t}{RESET}"),
+                    "english" => format!("unification failed:\nlhs: {YELLOW}{lhs_t}{RESET}\nrhs: {YELLOW}{rhs_t}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn re_unification_error(
@@ -599,10 +671,19 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
             (None, Some(r)) => r,
             (None, None) => Location::Unknown,
         };
-        Self::new(ErrorCore::new(0, TypeError, loc, switch_lang!(
-            format!("re-unification failed:\nlhs: {YELLOW}{lhs_t}{RESET}\nrhs: {YELLOW}{rhs_t}{RESET}"),
-            format!("型の再単一化に失敗しました:\n左辺: {YELLOW}{lhs_t}{RESET}\n右辺: {YELLOW}{rhs_t}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("型の再単一化に失敗しました:\n左辺: {YELLOW}{lhs_t}{RESET}\n右辺: {YELLOW}{rhs_t}{RESET}"),
+                    "english" => format!("re-unification failed:\nlhs: {YELLOW}{lhs_t}{RESET}\nrhs: {YELLOW}{rhs_t}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn subtyping_error(
@@ -618,17 +699,35 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
             (None, Some(r)) => r,
             (None, None) => Location::Unknown,
         };
-        Self::new(ErrorCore::new(0, TypeError, loc, switch_lang!(
-            format!("subtype constraints cannot be satisfied:\nsubtype: {YELLOW}{sub_t}{RESET}\nsupertype: {YELLOW}{sup_t}{RESET}"),
-            format!("部分型制約を満たせません:\nサブタイプ: {YELLOW}{sub_t}{RESET}\nスーパータイプ: {YELLOW}{sup_t}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                loc,
+                switch_lang!(
+                    "japanese" => format!("部分型制約を満たせません:\nサブタイプ: {YELLOW}{sub_t}{RESET}\nスーパータイプ: {YELLOW}{sup_t}{RESET}"),
+                    "english" => format!("subtype constraints cannot be satisfied:\nsubtype: {YELLOW}{sub_t}{RESET}\nsupertype: {YELLOW}{sup_t}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn pred_unification_error(lhs: &Predicate, rhs: &Predicate, caused_by: Str) -> Self {
-        Self::new(ErrorCore::new(0, TypeError, Location::Unknown, switch_lang!(
-            format!("predicate unification failed:\nlhs: {YELLOW}{lhs}{RESET}\nrhs: {YELLOW}{rhs}{RESET}"),
-            format!("述語式の単一化に失敗しました:\n左辺: {YELLOW}{lhs}{RESET}\n右辺: {YELLOW}{rhs}{RESET}")
-        ), None), caused_by)
+        Self::new(
+            ErrorCore::new(
+                0,
+                TypeError,
+                Location::Unknown,
+                switch_lang!(
+                    "japanese" => format!("述語式の単一化に失敗しました:\n左辺: {YELLOW}{lhs}{RESET}\n右辺: {YELLOW}{rhs}{RESET}"),
+                    "english" => format!("predicate unification failed:\nlhs: {YELLOW}{lhs}{RESET}\nrhs: {YELLOW}{rhs}{RESET}"),
+                ),
+                None,
+            ),
+            caused_by,
+        )
     }
 
     pub fn has_effect<S: Into<Str>>(expr: &Expr, caused_by: S) -> Self {
@@ -638,8 +737,8 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
                 HasEffect,
                 expr.loc(),
                 switch_lang!(
-                    format!("this expression causes a side-effect"),
-                    format!("この式には副作用があります")
+                    "japanese" => format!("この式には副作用があります"),
+                    "english" => format!("this expression causes a side-effect"),
                 ),
                 None,
             ),
@@ -659,14 +758,14 @@ passed keyword args:    {RED}{kw_args_len}{RESET}"
                 MoveError,
                 name_loc,
                 switch_lang!(
-                    format!(
+                    "japanese" => format!(
+                        "{RED}{name}{RESET}は{}行目ですでに移動されています",
+                        moved_loc.ln_begin().unwrap()
+                    ),
+                    "english" => format!(
                         "{RED}{name}{RESET} was moved in line {}",
                         moved_loc.ln_begin().unwrap()
                     ),
-                    format!(
-                        "{RED}{name}{RESET}は{}行目ですでに移動されています",
-                        moved_loc.ln_begin().unwrap()
-                    )
                 ),
                 None,
             ),
diff --git a/compiler/erg_compiler/lower.rs b/compiler/erg_compiler/lower.rs
index f15d0bde..b204821f 100644
--- a/compiler/erg_compiler/lower.rs
+++ b/compiler/erg_compiler/lower.rs
@@ -62,13 +62,13 @@ impl ASTLowerer {
                 expr.loc(),
                 self.ctx.name.clone(),
                 switch_lang!(
-                    "the evaluation result of the expression is not used",
-                    "式の評価結果が使われていません",
+                    "japanese" => "式の評価結果が使われていません",
+                    "english" => "the evaluation result of the expression is not used",
                 ),
                 Some(
                     switch_lang!(
-                        "if you don't use the value, use `discard` function",
-                        "値を使わない場合は、discard関数を使用してください",
+                        "japanese" => "値を使わない場合は、discard関数を使用してください",
+                        "english" => "if you don't use the value, use `discard` function",
                     )
                     .into(),
                 ),
diff --git a/compiler/erg_parser/Cargo.toml b/compiler/erg_parser/Cargo.toml
index 3753d02a..6c6af346 100644
--- a/compiler/erg_parser/Cargo.toml
+++ b/compiler/erg_parser/Cargo.toml
@@ -12,6 +12,8 @@ homepage = "https://erg-lang.github.io/"
 [features]
 debug = [ "erg_common/debug" ]
 japanese = [ "erg_common/japanese" ]
+simplified_chinese = [ "erg_common/simplified_chinese" ]
+traditional_chinese = [ "erg_common/traditional_chinese" ]
 
 [dependencies]
 erg_common = { version = "0.2.5", path = "../erg_common" }
diff --git a/compiler/erg_parser/error.rs b/compiler/erg_parser/error.rs
index 694beeb9..a7874e48 100644
--- a/compiler/erg_parser/error.rs
+++ b/compiler/erg_parser/error.rs
@@ -21,10 +21,16 @@ impl LexError {
     }
 
     pub fn compiler_bug(errno: usize, loc: Location, fn_name: &str, line: u32) -> Self {
-        Self::new(ErrorCore::new(errno, CompilerSystemError, loc, switch_lang!(
-            format!("this is a bug of the Erg compiler, please report it to https://github.com/mtshiba/erg\ncaused from: {fn_name}:{line}"),
-            format!("これはErg compilerのバグです、開発者に報告して下さい (https://github.com/mtshiba/erg)\n{fn_name}:{line}より発生")
-        ), None))
+        Self::new(ErrorCore::new(
+            errno,
+            CompilerSystemError,
+            loc,
+            switch_lang!(
+                "japanese" => format!("これはErg compilerのバグです、開発者に報告して下さい (https://github.com/mtshiba/erg)\n{fn_name}:{line}より発生"),
+                "english" => format!("this is a bug of the Erg compiler, please report it to https://github.com/mtshiba/erg\ncaused from: {fn_name}:{line}"),
+            ),
+            None,
+        ))
     }
 
     pub fn feature_error(errno: usize, loc: Location, name: &str) -> Self {
@@ -33,8 +39,9 @@ impl LexError {
             FeatureError,
             loc,
             switch_lang!(
-                format!("this feature({name}) is not implemented yet"),
-                format!("この機能({name})はまだ正式に提供されていません")
+                "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+                "simplified_chinese" => format!("该功能({name})还没有正式提供"),
+                "english" => format!("this feature({name}) is not implemented yet"),
             ),
             None,
         ))
@@ -45,7 +52,11 @@ impl LexError {
             errno,
             SyntaxError,
             loc,
-            switch_lang!("invalid syntax", "不正な構文です"),
+            switch_lang!(
+                "japanese" => "不正な構文です",
+                "simplified_chinese" => "无效的语法",
+                "english" => "invalid syntax",
+            ),
             None,
         ))
     }
diff --git a/compiler/erg_parser/lex.rs b/compiler/erg_parser/lex.rs
index 456a2b68..9c35e5fe 100644
--- a/compiler/erg_parser/lex.rs
+++ b/compiler/erg_parser/lex.rs
@@ -266,8 +266,8 @@ impl Lexer /*<'a>*/ {
                     0,
                     comment.loc(),
                     switch_lang!(
-                        "invalid unicode character (bi-directional override) in comments",
-                        "不正なユニコード文字(双方向オーバーライド)がコメント中に使用されています"
+                        "japanese" => "不正なユニコード文字(双方向オーバーライド)がコメント中に使用されています",
+                        "english" => "invalid unicode character (bi-directional override) in comments",
                     ),
                     None,
                 ));
@@ -298,7 +298,10 @@ impl Lexer /*<'a>*/ {
             Some(Err(LexError::syntax_error(
                 0,
                 space.loc(),
-                switch_lang!("invalid indent", "インデントが不正です"),
+                switch_lang!(
+                    "japanese" => "インデントが不正です",
+                    "english" => "invalid indent",
+                ),
                 None,
             )))
         } else if self.prev_token.is(Newline) {
@@ -317,11 +320,14 @@ impl Lexer /*<'a>*/ {
             return Some(Err(LexError::syntax_error(
                 0,
                 token.loc(),
-                switch_lang!("indentation is too deep", "インデントが深すぎます"),
+                switch_lang!(
+                    "japanese" => "インデントが深すぎます",
+                    "english" => "indentation is too deep",
+                ),
                 Some(
                     switch_lang!(
-                        "The code is too complicated. Please split the process.",
-                        "コードが複雑すぎます。処理を分割してください"
+                        "japanese" => "コードが複雑すぎます。処理を分割してください",
+                        "english" => "The code is too complicated. Please split the process",
                     )
                     .into(),
                 ),
@@ -359,7 +365,10 @@ impl Lexer /*<'a>*/ {
                     Some(Err(LexError::syntax_error(
                         0,
                         invalid_dedent.loc(),
-                        switch_lang!("invalid indent", "インデントが不正です"),
+                        switch_lang!(
+                            "japanese" => "インデントが不正です",
+                            "english" => "invalid indent",
+                        ),
                         None,
                     )))
                 }
@@ -529,9 +538,9 @@ impl Lexer /*<'a>*/ {
                         0,
                         token.loc(),
                         switch_lang!(
-                        "invalid unicode character (bi-directional override) in string literal",
-                        "不正なユニコード文字(双方向オーバーライド)が文字列中に使用されています"
-                    ),
+                            "japanese" => "不正なユニコード文字(双方向オーバーライド)が文字列中に使用されています",
+                            "english" => "invalid unicode character (bi-directional override) in string literal",
+                        ),
                         None,
                     ));
                 }
@@ -542,8 +551,8 @@ impl Lexer /*<'a>*/ {
             0,
             token.loc(),
             switch_lang!(
-                "the string is not closed by \"",
-                "文字列が\"によって閉じられていません"
+                "japanese" => "文字列が\"によって閉じられていません",
+                "english" => "the string is not closed by \"",
             ),
             None,
         ))
@@ -590,7 +599,10 @@ impl Iterator for Lexer /*<'a>*/ {
                         Some(Err(LexError::syntax_error(
                             0,
                             token.loc(),
-                            switch_lang!("no such operator: <.", "<.という演算子はありません"),
+                            switch_lang!(
+                                "japanese" => "<.という演算子はありません",
+                                "english" => "no such operator: <.",
+                            ),
                             None,
                         )))
                     }
@@ -764,8 +776,17 @@ impl Iterator for Lexer /*<'a>*/ {
                 Some(Err(LexError::syntax_error(
                     0,
                     token.loc(),
-                    switch_lang!("cannot use a tab as a space", "タブ文字は使用できません"),
-                    Some(switch_lang!("use spaces", "スペースを使用してください").into()),
+                    switch_lang!(
+                        "japanese" => "タブ文字は使用できません",
+                        "english" => "cannot use a tab as a space",
+                    ),
+                    Some(
+                        switch_lang!(
+                            "japanese" => "スペース( )を使用してください",
+                            "english" => "use spaces ( )",
+                        )
+                        .into(),
+                    ),
                 )))
             }
             // TODO:
@@ -788,8 +809,8 @@ impl Iterator for Lexer /*<'a>*/ {
                                 0,
                                 token.loc(),
                                 switch_lang!(
-                                    format!("`{}` cannot be defined by user", &token.content),
-                                    format!("`{}`はユーザー定義できません", &token.content)
+                                    "japanese" => format!("`{}`はユーザー定義できません", &token.content),
+                                    "english" => format!("`{}` cannot be defined by user", &token.content),
                                 ),
                                 None,
                             )));
@@ -802,8 +823,8 @@ impl Iterator for Lexer /*<'a>*/ {
                     0,
                     token.loc(),
                     switch_lang!(
-                        format!("back quotes (`) not closed"),
-                        format!("バッククォート(`)が閉じられていません")
+                        "japanese" => format!("バッククォート(`)が閉じられていません"),
+                        "english" => format!("back quotes (`) not closed"),
                     ),
                     None,
                 )))
@@ -819,8 +840,8 @@ impl Iterator for Lexer /*<'a>*/ {
                     0,
                     token.loc(),
                     switch_lang!(
-                        format!("invalid character: '{invalid}'"),
-                        format!("この文字は使用できません: '{invalid}'")
+                        "japanese" => format!("この文字は使用できません: '{invalid}'"),
+                        "english" => format!("invalid character: '{invalid}'"),
                     ),
                     None,
                 )))
diff --git a/compiler/erg_parser/parse.rs b/compiler/erg_parser/parse.rs
index 8dc2f35b..a8612d8c 100644
--- a/compiler/erg_parser/parse.rs
+++ b/compiler/erg_parser/parse.rs
@@ -439,7 +439,10 @@ impl Parser {
             let err = ParseError::syntax_error(
                 0,
                 loc,
-                switch_lang!("failed to parse a block", "ブロックの解析に失敗しました"),
+                switch_lang!(
+                    "japanese" => "ブロックの解析に失敗しました",
+                    "english" => "failed to parse a block",
+                ),
                 None,
             );
             Err(err)
@@ -496,8 +499,8 @@ impl Parser {
                     0,
                     self.peek().unwrap().loc(),
                     switch_lang!(
-                        "Cannot use type bounds in a declaration of a variable",
-                        "変数宣言で型制約は使えません"
+                        "japanese" => "変数宣言で型制約は使えません",
+                        "english" => "Cannot use type bounds in a declaration of a variable",
                     ),
                     None,
                 );
@@ -509,16 +512,6 @@ impl Parser {
                     self.skip();
                     Some(self.try_reduce_type_spec()?)
                 } else {
-                    if self.cur_is(Colon) {
-                        self.warns.push(ParseError::syntax_warning(0, name.loc(), switch_lang!(
-                                "Since it is obvious that the variable is of type `Type`, there is no need to specify the type.",
-                                "変数がType型であることは明らかなので、型指定は不要です。"
-                            ), Some(switch_lang!(
-                                "Are you sure you're not confusing it with subclass declaration (<:)?",
-                                "サブクラスの宣言(<:)ではありませんか?"
-                            ).into())
-                        ));
-                    }
                     None
                 }
             } else if self.cur_is(Colon) {
@@ -663,8 +656,8 @@ impl Parser {
                         0,
                         t.loc(),
                         switch_lang!(
-                            "Binary operators cannot be used in left-values",
-                            "左辺値の中で中置演算子は使えません"
+                            "japanese" => "左辺値の中で中置演算子は使えません",
+                            "english" => "Binary operators cannot be used in left-values",
                         ),
                         None,
                     );
@@ -754,9 +747,10 @@ impl Parser {
                             return Err(ParseError::syntax_error(
                                 0,
                                 param.loc(),
+                                // TODO: switch_lang!
                                 "non-default argument follows default argument",
                                 None,
-                            ))
+                            ));
                         }
                         (false, false) => {
                             non_default_params.push(param);
@@ -768,8 +762,8 @@ impl Parser {
                         0,
                         t.loc(),
                         switch_lang!(
-                            "Binary operators cannot be used in parameters",
-                            "仮引数の中で中置演算子は使えません"
+                            "japanese" => "仮引数の中で中置演算子は使えません",
+                            "english" => "Binary operators cannot be used in parameters",
                         ),
                         None,
                     );
@@ -1015,18 +1009,21 @@ impl Parser {
 
     fn validate_const_expr(&mut self, expr: Expr) -> ParseResult<ConstExpr> {
         match expr {
-            Expr::Lit(l) =>  Ok(ConstExpr::Lit(l)),
+            Expr::Lit(l) => Ok(ConstExpr::Lit(l)),
             Expr::Accessor(Accessor::Local(local)) => {
                 let local = ConstLocal::new(local.symbol);
                 Ok(ConstExpr::Accessor(ConstAccessor::Local(local)))
             }
             // TODO: App, Array, Record, BinOp, UnaryOp,
-            other => {
-                Err(ParseError::syntax_error(0, other.loc(), switch_lang!(
-                    "this expression is not computable at the compile-time, so cannot used as a type-argument",
-                    "この式はコンパイル時計算できないため、型引数には使用できません",
-                ), None))
-            }
+            other => Err(ParseError::syntax_error(
+                0,
+                other.loc(),
+                switch_lang!(
+                    "japanese" => "この式はコンパイル時計算できないため、型引数には使用できません",
+                    "english" => "this expression is not computable at the compile-time, so cannot used as a type-argument",
+                ),
+                None,
+            )),
         }
     }
 
diff --git a/doc/EN/dev_guide/i18n_messages.md b/doc/EN/dev_guide/i18n_messages.md
new file mode 100644
index 00000000..7736ce80
--- /dev/null
+++ b/doc/EN/dev_guide/i18n_messages.md
@@ -0,0 +1,59 @@
+# Multilingualization of Messages
+
+Erg is working on making all messages (start, option, doc, hint, warning, error messages, etc.) multilingual within the language.
+This project is open to anyone without detailed knowledge of Rust or Erg. Your participation is always welcome.
+
+Here is how to translate them.
+
+## Search `switch_lang!`
+
+In the Erg source code, look for the item `switch_lang!` (use grep or your editor's search function).
+You should find something like this:
+
+```rust
+switch_lang!(
+    "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+    "english" => format!("this feature({name}) is not implemented yet"),
+),
+```
+
+This message is currently supported only in Japanese and English. Let's add a simplified Chinese message as a test.
+
+## Add a New Message
+
+Add translated messages as you see the content in other languages. Don't forget the comma (`,`) last.
+
+```rust
+switch_lang!(
+    "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+    "simplified_chinese" => format!("该功能({name})还没有正式提供"),
+    "english" => format!("this feature({name}) is not implemented yet"),
+),
+```
+
+Note that English is the default and must come last.
+The `{name}` part is a Rust formatting feature that allows you to embed the contents of a variable (`name`) into a string.
+
+## Build
+
+Now, let's build with the `--features simplified_chinese` option.
+
+<img src="../../../assets/screenshot_i18n_messages.png" alt='screenshot_i18n_messages'>
+
+We did it!
+
+## FAQ
+
+Q: What does a specification like `{RED}{foo}{RESET}` mean?
+A: {RED} and subsequent letters will be displayed in red. {RESET} will restore the color.
+
+Q: If I want to add my language, how do I replace the `"simplified_chinese" =>` part?
+
+The following languages are currently supported:
+
+* "english" (default)
+* "japanese"
+* "simplified_chinese"
+* "traditional_chinese"
+
+If you would like to add languages other than these, please make a request.
diff --git a/doc/JA/dev_guide/i18n_messages.md b/doc/JA/dev_guide/i18n_messages.md
new file mode 100644
index 00000000..675d3f3d
--- /dev/null
+++ b/doc/JA/dev_guide/i18n_messages.md
@@ -0,0 +1,58 @@
+# Multilingualization of Messages
+
+Ergはメッセージ(スタート、オプション、ドキュメント、ヒント、警告、エラーメッセージなど)の多言語化を進めています。
+このプロジェクトは、RustやErgの詳しい知識がなくても参加することができます。ぜひ協力をお願いします。
+
+以下に、多言語化の方法を説明します。
+
+## `switch_lang!`を探す
+
+Ergのソースコードの中で、`switch_lang!`という項目を探します(grepやエディタの検索機能を使ってください)。
+以下のようなものが見つかるはずです。
+
+```rust
+switch_lang!(
+    "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+    "english" => format!("this feature({name}) is not implemented yet"),
+),
+```
+
+このメッセージは現在、日本語と英語のみでサポートされています。試しに簡体字のメッセージを追加してみましょう。
+
+## メッセージを追加する
+
+他の言語の内容を見ながら、翻訳されたメッセージを追加してください。最後にカンマ(`,`)を忘れないでください。
+
+```rust
+switch_lang!(
+    "japanese" => format!("この機能({name})はまだ正式に提供されていません"),
+    "simplified_chinese" => format!("该功能({name})还没有正式提供"),
+    "english" => format!("this feature({name}) is not implemented yet"),
+),
+```
+
+なお、英語はデフォルトであり、必ず最後に来るようにします。
+`{name}` の部分は Rust のフォーマット機能で、変数の内容 (`name`) を文字列に埋め込むことができます。
+
+## Build
+
+では、`--features simplified_chinese` オプションを付けてビルドしてみましょう。
+
+<img src="../../../assets/screenshot_i18n_messages.png" alt='screenshot_i18n_messages'>
+
+やりましたね!
+
+## FAQ
+
+Q: `{RED}{foo}{RESET}` のような指定は何を意味するのでしょうか？
+A: {RED}以降が赤色で表示されます。{RESET}で色を元に戻します。
+
+Q: 自分の言語を追加したい場合、`"simplified_chinese" =>`の部分はどのように置き換えればよいですか？
+A: 現在、以下の言語がサポートされています。
+
+* "english" (デフォルト)
+* "japanese" (日本語)
+* "simplified_chinese" (簡体字中国語)
+* "traditional_chinese"（繁体字中国語）
+
+これら以外の言語を追加したい場合は、リクエストしてください。