code: def from_xx(cls, xx):
          
          xx = str(xx).lower()
          if xx is 'unknown':
              return UnknownLanguage(xx)
          try:
              return cls._from_xyz('ISO639', xx)
          except NotALanguageException:
              log.warning('Unknown ISO639: {}'.format(xx))
              return UnknownLanguage(xx)
docs: Create a new Language instance from a ISO639 string
          :param xx: ISO639 as string
          :return: Language instance with instance.xx() == xx if xx is valid else instance of UnknownLanguage