Make external entities translatable

In ✨ Make external entities language aware RTBC we made external entities language aware.

A next improvement would be to make external entities translatable.

I'm working on a project where:

1. The external entity should have a "Translate"-tab
2. The different translations can be viewed individually (e.g. /en/my-external-entity/123, /de/my-external-entity/123, ...)
3. Each translation can be saved
4. Each translation can be annotated

Most of these are pretty default functionality with core entity types (such as node).