[WIP] Add requirements for language fall backs.

> Change deserializer to drop TermFallback silently (I don't like this, although it is at least better than serializing them as Term and silently adding them to the database. Should investigate how hard it is to output an error in the API based on an exception in the deserializer.)

Note that I have no strong preference on this topic, since it's a question of what is best in the web API.

> FilterLanguages

That is a function name, not a class name.