Hm, you might want to ask for help in the #plugin:translator category.
No, I don’t think so.
Yes, such sensitivity to user locale makes sense, especially for the banner post, because you can only have one. Unfortunately, it’s not possible at the moment. As a workaround, I would suggest you simply compose two separate topics and perhaps include a flag emoji in the topic title. That should work for all scenarios except for banners.
If you want to develop this into a feature request, I think the easiest way to implement this would be to make it possible to hide topics based on user locale (or perhaps: show based on user locale). That way you would simply hide the irrelevant versions of those topics for users with certain locales.