Activate a translation for lang.

If lang is None, then the language of locale.getdefaultlocale() is used.
If the translation file does not exist, the original messages will be used.

def activate(lang=None):
    """Activate a translation for lang.

If lang is None, then the language of locale.getdefaultlocale() is used.
If the translation file does not exist, the original messages will be used.
"""
    if lang is None:
        lang = locale.getlocale()[0]
    tr = gettext.translation("argparse", os.path.join(locpath, "locale"),
                             [lang], fallback=True)
    argparse._ = tr.gettext
    argparse.ngettext = tr.ngettext