I don't know the extension, but from the documentation, the interface is only mandatory to found out if the entity is translatable. However, I would like to not accept this situation as it ;) I am pretty sure it will be possible to build an array of translatable entities and activate this feature for those entities.

The idea behind this is to activate the entity localization "on demand" with a configuration settings. So we can reuse an entity and make it translatable.