'Updates default_locale if it\'s different and matches one of our
supported locales.
Returns tuple of (old_locale, new_locale) if updated. Otherwise None.'

def update_default_locale(self, locale):

    old_locale = self.default_locale
    locale = find_language(locale)
    if (locale and (locale != old_locale)):
        self.update(default_locale=locale)
        return (old_locale, locale)
    return None
