I worked again on the patch. I opened new issues to prepare the new mbcs codec:
 - #8966: ctypes: remove implicit conversion between unicode and bytes
 - #8967: Create PyErr_GetWindowsMessage() function
 - #8969: Windows: use (mbcs in) strict mode to encode/decode filenames, and enable os.fsencode()

#8967 can be used to get the translated message of a mbcs encode error. PyErr_GetWindowsMessage() returns a PyUnicodeObject, whereas make_translate_exception() and PyUnicodeTranslateError_SetReason() expect a "char*". Another patch is requied: translate_reason_unicode.patch (attached to this issue, not tested). But I don't think that the message is very important for now :-)

#8784 (tarfile/Windows: Don't use mbcs as the default encoding) is still open.