pythongh-87281: Improve documentation for locale.setlocale() and locale.getlocale() (pythonGH-137313)

Add a section explaining the locale name formats.

Author: serhiy-storchaka

Doc/library/locale.rst

@@ -34,12 +34,17 @@ The :mod:`locale` module defines the following exception and functions:
 
    If *locale* is given and not ``None``, :func:`setlocale` modifies the locale
    setting for the *category*. The available categories are listed in the data
-   description below. *locale* may be a string, or an iterable of two strings
-   (language code and encoding). If it's an iterable, it's converted to a locale
-   name using the locale aliasing engine. An empty string specifies the user's
+   description below. *locale* may be a :ref:`string <locale_name>`, or a pair,
+   language code and encoding. An empty string specifies the user's
    default settings. If the modification of the locale fails, the exception
    :exc:`Error` is raised. If successful, the new locale setting is returned.
 
+   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.
+   The language code and encoding can be ``None``.
+
    If *locale* is omitted or ``None``, the current setting for *category* is
    returned.
 
@@ -345,22 +350,26 @@ The :mod:`locale` module defines the following exception and functions:
    ``'LANG'``.  The GNU gettext search path contains ``'LC_ALL'``,
    ``'LC_CTYPE'``, ``'LANG'`` and ``'LANGUAGE'``, in that order.
 
-   Except for the code ``'C'``, the language code corresponds to :rfc:`1766`.
-   *language code* and *encoding* may be ``None`` if their values cannot be
+   The language code has the same format as a :ref:`locale name <locale_name>`,
+   but without encoding and ``@``-modifier.
+   The language code and encoding may be ``None`` if their values cannot be
    determined.
+   The "C" locale is represented as ``(None, None)``.
 
    .. deprecated-removed:: 3.11 3.15
 
 
 .. function:: getlocale(category=LC_CTYPE)
 
-   Returns the current setting for the given locale category as sequence containing
-   *language code*, *encoding*. *category* may be one of the :const:`!LC_\*` values
-   except :const:`LC_ALL`.  It defaults to :const:`LC_CTYPE`.
+   Returns the current setting for the given locale category as a tuple containing
+   the language code and encoding. *category* may be one of the :const:`!LC_\*`
+   values except :const:`LC_ALL`.  It defaults to :const:`LC_CTYPE`.
 
-   Except for the code ``'C'``, the language code corresponds to :rfc:`1766`.
-   *language code* and *encoding* may be ``None`` if their values cannot be
+   The language code has the same format as a :ref:`locale name <locale_name>`,
+   but without encoding and ``@``-modifier.
+   The language code and encoding may be ``None`` if their values cannot be
    determined.
+   The "C" locale is represented as ``(None, None)``.
 
 
 .. function:: getpreferredencoding(do_setlocale=True)
@@ -615,6 +624,61 @@ whose high bit is set (i.e., non-ASCII bytes) are never converted or considered
 part of a character class such as letter or whitespace.
 
 
+.. _locale_name:
+
+Locale names
+------------
+
+The format of the locale name is platform dependent, and the set of supported
+locales can depend on the system configuration.
+
+On Posix platforms, it usually has the format [1]_:
+
+.. productionlist:: locale_name
+   : language ["_" territory] ["." charset] ["@" modifier]
+
+where *language* is a two- or three-letter language code from `ISO 639`_,
+*territory* is a two-letter country or region code from `ISO 3166`_,
+*charset* is a locale encoding, and *modifier* is a script name,
+a language subtag, a sort order identifier, or other locale modifier
+(for example, "latin", "valencia", "stroke" and "euro").
+
+On Windows, several formats are supported. [2]_ [3]_
+A subset of `IETF BCP 47`_ tags:
+
+.. productionlist:: locale_name
+   : language ["-" script] ["-" territory] ["." charset]
+   : language ["-" script] "-" territory "-" modifier
+
+where *language* and *territory* have the same meaning as in Posix,
+*script* is a four-letter script code from `ISO 15924`_,
+and *modifier* is a language subtag, a sort order identifier
+or custom modifier (for example, "valencia", "stroke" or "x-python").
+Both hyphen (``'-'``) and underscore (``'_'``) separators are supported.
+Only UTF-8 encoding is allowed for BCP 47 tags.
+
+Windows also supports locale names in the format:
+
+.. productionlist:: locale_name
+   : language ["_" territory] ["." charset]
+
+where *language* and *territory* are full names, such as "English" and
+"United States", and *charset* is either a code page number (for example, "1252")
+or UTF-8.
+Only the underscore separator is supported in this format.
+
+The "C" locale is supported on all platforms.
+
+.. _ISO 639: https://www.iso.org/iso-639-language-code
+.. _ISO 3166: https://www.iso.org/iso-3166-country-codes.html
+.. _IETF BCP 47: https://www.rfc-editor.org/info/bcp47
+.. _ISO 15924: https://www.unicode.org/iso15924/
+
+.. [1] `IEEE Std 1003.1-2024; 8.2 Internationalization Variables <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html#tag_08_02>`_
+.. [2] `UCRT Locale names, Languages, and Country/Region strings <https://learn.microsoft.com/en-us/cpp/c-runtime-library/locale-names-languages-and-country-region-strings>`_
+.. [3] `Locale Names <https://learn.microsoft.com/en-us/windows/win32/intl/locale-names>`_
+
+
 .. _embedding-locale:
 
 For extension writers and programs that embed Python