@BeAware as far as I'm aware of mastodon clients try to figure this out by themself without setting it explicitly. For example this one should be marked as English, while my default is german.
The problem is with 3rd party clients that do not support it or do a bad job with it. This kinda defeats the purpose of this whole feature.
Yeah, server side is probably a bad idea that isn't going to work, but how could one improve this situation?