As I said this is not show stopper at all. You really want to see me saying things that I don't.
I just inform about the problem with current approach, and did warn about the impact that's all, if there's no way to do it correctly, then a workaround is of course welcome as long as this is not breaking things or current conventions of course.
And since we had long discussion about making Kodi code better, I find normal to report this and alert about future impacts or unseen ones.
From remote point of view fixed ID sounded better, because it's easy to intercept and translate at display time. The GetMissing... also does the trick at the expanse of one more call. (And IMO fixed id sounds easier for GUI / addons translations too but I have no real clue about that)
Having the API returning the data in English is better also in the same principle as API not dependent of Kodi settings, but since I know nothing about Kodi GUI maybe this is not possible to distinguish the 2 cases and translate for GUI.
(Think about dates returned by JSON they are all standard and not formatted according to Kodi language, time are on 24h, Global.Weekday are in English, ....)
About profiles :
http://kodi.wiki/view/Profiles, profiles can have shared media info and sources, and not always start from scratch with a new DB. This is more common than central DB I think.