code: def save_existing(self, form, instance, commit=True):
          
          self._prepare_multilingual_object(instance, form)
          return forms.save_instance(form, instance, exclude=[self._pk_field.name], commit=commit)
docs: NOTE: save_new method is completely overridden here, there's no
          other way to pretend double save otherwise. Just assign translated data
          to object