Migrate to modern win32/winrt API for `localemodule`

Feature or enhancement

Proposal:

AFAIK, CPython has dropped support for Windows 7 since 3.9.0, while _localemodule.c contains compatibility fix for Windows NT pre-4.0. Which flag was introduced since Windows 98.

Here I listed some modern way to get locale name in BCP-47 / RFC 1766 format, is it possible to replace the legacy hex code mapping?

• GlobalizationPreferences since Windows 10 (build 10240)
• GetUserDefaultLocaleName since Windows Vista / Windows Server 2008

import ctypes
from ctypes import wintypes
from locale import getdefaultlocale, getlocale, LC_CTYPE
import warnings

# Introduced since Windows 10, BCP-47
# depends on:
# - winrt-windows-system-userprofile
# - winrt-windows-foundation
# - winrt-windows-foundation-collections
from winrt.windows.system.userprofile import GlobalizationPreferences

warnings.filterwarnings("ignore", category=DeprecationWarning)

# Load kernel32.dll
kernel32 = ctypes.WinDLL("kernel32", use_last_error=True)

# Introduced since Windows Vista, RFC 1766
# Define GetUserDefaultLocaleName signature
# int GetUserDefaultLocaleName(LPWSTR lpLocaleName, int cchLocaleName);
kernel32.GetUserDefaultLocaleName.argtypes = [wintypes.LPWSTR, ctypes.c_int]
kernel32.GetUserDefaultLocaleName.restype = ctypes.c_int

# Allocate buffer for locale name
LOCALE_NAME_MAX_LENGTH = 85  # per Windows docs


def user_default_locale() -> str:
    """
    Get user default locale encoded in BCP-47.
    """
    buffer = ctypes.create_unicode_buffer(LOCALE_NAME_MAX_LENGTH)

    # Call the function
    result = kernel32.GetUserDefaultLocaleName(buffer, LOCALE_NAME_MAX_LENGTH)

    if result == 0:
        # Call failed
        error_code = ctypes.get_last_error()
        raise ctypes.WinError(error_code)

    return buffer.value


def bcp47():
    return GlobalizationPreferences.languages[0]

>>> getdefaultlocale()
('zh_CN', 'cp936')
>>> getlocale(LC_CTYPE)
('Chinese (Simplified)_China', '936')
>>> user_default_locale()
'zh-CN'
>>> bcp47()
'zh-Hans-CN'

Has this already been discussed elsewhere?

Yes

Links to previous discussion of this feature:

gh-90817 gh-130796 https://github.com/python/cpython/issues/123853#issuecomment-3132027944

Start with Modules/_localemodule.c, especially the compatibility mapping for Windows NT pre-4.0. Review the linked discussions in gh-90817, gh-130796, and issue 123853, then compare GetUserDefaultLocaleName with GlobalizationPreferences. The work is complete only once the project agrees on a modern API and the legacy hex-code mapping is replaced consistently.