Update field in the correct language/entity, not the original

In the EditableFieldsForm::submitForm the entity is reloaded but the language is not preserved.
So we need to make sure, we update the respective translation of the entity.

Additionally, as we can do multiple updates on the same entity for just a specific field, we should not create a new revision every-time, hence the addition of $entity->setSyncing(TRUE); before the save.