Taggit problem with override

gistfile1.py

class I18NTag(TagBase):
    """Extend Taggit's Tag:

    * Add a language field
    * slug unique with language field
    """
    language = models.CharField(max_length=5, choices=settings.LANGUAGES,
                                blank=True, default=settings.LANGUAGE_CODE)
    slug = models.SlugField(unique=False, max_length=100)

    class Meta:
        unique_together = ('language', 'slug')