Fixed #27395 -- Added sitemap 'alternates' generation.

Updated the sitemap generator and default template to optionally
include link elements with hreflang attribute to alternate language
URLs.

docs/ref/contrib/sitemaps.txt

@@ -252,6 +252,40 @@ Note:
         be generated using all of your :setting:`LANGUAGES`. The default is
         ``False``.
 
+    .. attribute:: Sitemap.languages
+
+        .. versionadded:: 3.2
+
+        **Optional.**
+
+        A :term:`sequence` of :term:`language codes<language code>` to use for
+        generating alternate links when :attr:`~Sitemap.i18n` is enabled.
+        Defaults to :setting:`LANGUAGES`.
+
+    .. attribute:: Sitemap.alternates
+
+        .. versionadded:: 3.2
+
+        **Optional.**
+
+        A boolean attribute. When used in conjunction with
+        :attr:`~Sitemap.i18n` generated URLs will each have a list of alternate
+        links pointing to other language versions using the `hreflang
+        attribute`_. The default is ``False``.
+
+        .. _hreflang attribute: https://support.google.com/webmasters/answer/189077
+
+    .. attribute:: Sitemap.x_default
+
+        .. versionadded:: 3.2
+
+        **Optional.**
+
+        A boolean attribute. When ``True`` the alternate links generated by
+        :attr:`~Sitemap.alternates` will contain a ``hreflang="x-default"``
+        fallback entry with a value of :setting:`LANGUAGE_CODE`. The default is
+        ``False``.
+
 Shortcuts
 =========
 
@@ -438,12 +472,22 @@ The variable ``urlset`` is a list of URLs that should appear in the
 sitemap. Each URL exposes attributes as defined in the
 :class:`~django.contrib.sitemaps.Sitemap` class:
 
+- ``alternates``
 - ``changefreq``
 - ``item``
 - ``lastmod``
 - ``location``
 - ``priority``
 
+The ``alternates`` attribute is available when :attr:`~Sitemap.i18n` and
+:attr:`~Sitemap.alternates` are enabled. It is a list of other language
+versions, including the optional :attr:`~Sitemap.x_default` fallback, for each
+URL. Each alternate is a dictionary with ``location`` and ``lang_code`` keys.
+
+.. versionchanged:: 3.2
+
+    The ``alternates`` attribute was added.
+
 The ``item`` attribute has been added for each URL to allow more flexible
 customization of the templates, such as `Google news sitemaps`_. Assuming
 Sitemap's :attr:`~Sitemap.items()` would return a list of items with