public string? get_locale_for_key (string group_name, string key, string? locale = null)
If calling get_locale_string or
get_locale_string_list with exactly the same
locale, the result of those functions will
have originally been tagged with the locale that is the result of this function.
a group name
a locale identifier or null
the locale from the file, or null if the key was not found or the entry in the file was was untranslated