Refs #30997 -- Removed HttpRequest.is_ajax() usage.

docs/releases/3.1.txt

@@ -388,6 +388,11 @@ Miscellaneous
   Django 3.1, the first request to any previously cached template fragment will
   be a cache miss.
 
+* The logic behind the decision to return a redirection fallback or a 204 HTTP
+  response from the :func:`~django.views.i18n.set_language` view is now based
+  on the ``Accept`` HTTP header instead of the ``X-Requested-With`` HTTP header
+  presence.
+
 * The compatibility imports of ``django.core.exceptions.EmptyResultSet`` in
   ``django.db.models.query``, ``django.db.models.sql``, and
   ``django.db.models.sql.datastructures`` are removed.

docs/topics/i18n/translation.txt

@@ -1803,9 +1803,18 @@ redirect to that URL will be performed. Otherwise, Django may fall back to
 redirecting the user to the URL from the ``Referer`` header or, if it is not
 set, to ``/``, depending on the nature of the request:
 
-* For AJAX requests, the fallback will be performed only if the ``next``
-  parameter was set. Otherwise a 204 status code (No Content) will be returned.
-* For non-AJAX requests, the fallback will always be performed.
+* If the request accepts HTML content (based on its ``Accept`` HTTP header),
+  the fallback will always be performed.
+
+* If the request doesn't accept HTML, the fallback will be performed only if
+  the ``next`` parameter was set. Otherwise a 204 status code (No Content) will
+  be returned.
+
+.. versionchanged:: 3.1
+
+    In older versions, the distinction for the fallback is based on whether the
+    ``X-Requested-With`` header is set to the value ``XMLHttpRequest``. This is
+    set by the jQuery ``ajax()`` method.
 
 Here's example HTML template code:
 

docs/topics/testing/tools.txt

@@ -159,11 +159,11 @@ Use the ``django.test.Client`` class to make requests.
 
             >>> c = Client()
             >>> c.get('/customers/details/', {'name': 'fred', 'age': 7},
-            ...       HTTP_X_REQUESTED_WITH='XMLHttpRequest')
+            ...       HTTP_ACCEPT='application/json')
 
-        ...will send the HTTP header ``HTTP_X_REQUESTED_WITH`` to the
-        details view, which is a good way to test code paths that use the
-        :meth:`django.http.HttpRequest.is_ajax()` method.
+        ...will send the HTTP header ``HTTP_ACCEPT`` to the details view, which
+        is a good way to test code paths that use the
+        :meth:`django.http.HttpRequest.accepts()` method.
 
         .. admonition:: CGI specification