Translatable ForeignKey cannot be assigned by id

Foreign keys in Django can normally be assigned by id, this way:
```python
instance.related_id = related.pk
```
However, translatable models lack descriptors for the `*_id`. Therefore, if, in the above example, `related` is a translatable foreign key, the assignment will not be captured and end up setting an attribute on the shared model. As it is not in the right place, the ForeignKey field in turn will not save it.