[translations] allow detached document with locale propagation to mapped associations

managed documents can only ever exist once per DocumentManager, and thus only in one language. for this reason we did not want to make the language propagate for mapped children and references (as this would change the content of documents in a hard to understand way - and loading anything in a different language again would still lose the language)

thus we should provide a findTranslation variant to get a detached translated document that fetches all associations as detached as well, keeping the locale setting.

there is no way to handle the write case, but this would cover the read case.