code: def get_available_languages(self, obj, formset):
          
          if obj:
              # Inlines dictate language code, not the parent model.
              # Hence, not looking at obj.get_available_languages(), but see what languages
              # are used by the inline objects that point to it.
              filter = {
                  'master__{0}'.format(formset.fk.name): obj
              }
              return self.model._parler_meta.root_model.objects.using(obj._state.db).filter(**filter) \
                     .values_list('language_code', flat=True).distinct().order_by('language_code')
          else:
              return self.model._parler_meta.root_model.objects.none()
docs: Fetching the available inline languages as queryset.