title: django-parler doesn't show tabs in admin
body: <p>For some reason I'm not seeing any language tabs when adding to the admin. I'm using Django 1.9.10. I was using django-hvad but decided to try parler. I have tried the same exact code in a fresh project and it worked but in my existing project it doesn't. Please note that I'm also using django-mptt. Tried parler in a separate model from the mptt model and vice versa.</p>
  
  <pre><code>settings/base.py
  
  # Translations
  gettext = lambda s: s
  LANGUAGES = (
      ('en', gettext('English')),
      ('ar', gettext('Arabic')),
  )
  
  LOCALE_PATHS = (
      os.path.join(BASE_DIR, "locale"),
  )
  
  # # Parler Configuration
  PARLER_LANGUAGES = {
      None: (
          {'code': 'ar', },
          {'code': 'en',},
      ),
      'default': {
          'fallback': 'ar',             # defaults to PARLER_DEFAULT_LANGUAGE_CODE
          'hide_untranslated': False,   # the default; let .active_translations()       return fallbacks too.
      }
  }
  PARLER_DEFAULT_LANGUAGE_CODE = 'ar'
  </code></pre>
  
  <p>Model</p>
  
  <pre><code>class Category(MPTTModel, TranslatableModel):
      slug = models.SlugField(max_length=50, unique=True, null=True, blank=True)
      translations = TranslatedFields(
          title = models.CharField(max_length=90, unique=True, null=True, blank=True)
      )
      timestamp = models.DateTimeField(auto_now_add=True, auto_now=False)
      parent = TreeForeignKey('self', null=True, blank=True, related_name='children')
  
      objects = CategoryManager()
  
      def __unicode__(self):
  
          return unicode(self.slug) or u''
  
      def get_absolute_url(self):
          return reverse("category:detail", kwargs={"slug": self.slug})
  
      class Meta:
          ordering = ["slug"]
          verbose_name = _("Category")
          verbose_name_plural = _("Categories")
  </code></pre>
  
  <p>admin.py</p>
  
  <pre><code>class CategoryAdminForm(MPTTAdminForm, TranslatableModelForm):
      pass
  
  
  class CategoryAdmin(TranslatableAdmin, MPTTModelAdmin):
      form = CategoryAdminForm
  
      def get_prepopulated_fields(self, request, obj=None):
          return {'slug': ('title',)}  # needed for translated fields
  
  
  admin.site.register(Category, CategoryAdmin)
  </code></pre>