Override _clone to preserve additional information needed by
        MultilingualModelQuerySet.

def _clone(self, klass=None, **kwargs):
        """
        Override _clone to preserve additional information needed by
        MultilingualModelQuerySet.
        """
        clone = super(MultilingualModelQuerySet, self)._clone(klass, **kwargs)
        clone._default_language = getattr(self, '_default_language', None)
        return clone