Add cocumentation.

Author: serhiy-storchaka

Doc/library/locale.rst

@@ -42,7 +42,7 @@ The :mod:`locale` module defines the following exception and functions:
    If *locale* is a pair, it is converted to a locale name using
    the locale aliasing engine.
    The language code has the same format as a :ref:`locale name <locale_name>`,
-   but without encoding and ``@``-modifier.
+   but without encoding.
    The language code and encoding can be ``None``.
 
    If *locale* is omitted or ``None``, the current setting for *category* is
@@ -58,6 +58,9 @@ The :mod:`locale` module defines the following exception and functions:
    specified in the :envvar:`LANG` environment variable).  If the locale is not
    changed thereafter, using multithreading should not cause problems.
 
+   .. verssionchanged:: next
+      Support language codes with ``@``-modifiers.
+
 
 .. function:: localeconv()
 
@@ -366,11 +369,14 @@ The :mod:`locale` module defines the following exception and functions:
    values except :const:`LC_ALL`.  It defaults to :const:`LC_CTYPE`.
 
    The language code has the same format as a :ref:`locale name <locale_name>`,
-   but without encoding and ``@``-modifier.
+   but without encoding.
    The language code and encoding may be ``None`` if their values cannot be
    determined.
    The "C" locale is represented as ``(None, None)``.
 
+   .. verssionchanged:: next
+      The language code can now contain a ``@``-modifier.
+
 
 .. function:: getpreferredencoding(do_setlocale=True)
 

Doc/whatsnew/3.15.rst

@@ -263,6 +263,15 @@ http.client
   (Contributed by Alexander Enrique Urieles Nieto in :gh:`131724`.)
 
 
+locale
+------
+
+* :func:`~locale.setlocale` now supports language codes with ``@``-modifiers.
+  ``@``-modifier are no longer silently removed in :func:`~locale.getlocale`,
+  but included in the language code.
+  (Contributed by Serhiy Storchaka in :gh:`137729`.)
+
+
 math
 ----
 

Misc/NEWS.d/next/Library/2025-08-14-00-00-12.gh-issue-137729.i9NSKP.rst

@@ -0,0 +1,3 @@
+:func:`locale.setlocale` now supports language codes with ``@``-modifiers.
+``@``-modifier are no longer silently removed in :func:`locale.getlocale`,
+but included in the language code.