Returns active language.

def active_language(self):
        """
        Returns active language.
        """
        # Current instance language (if user uses activate_language() method)
        if self._language is not None:
            return self._language

        # Current site language (translation.get_language())
        current = utils.get_language()
        if current in self.supported_languages:
            return current

        # Default language descriptor
        return self.default_language