The whole language setting is one of the things that annoys me about lemmy. Half the time I write a post and then hit "send" and then the post fails because the language setting is wrong for the community I'm in.
I really like the concept, just not the execution.
I haven't had that happen to me yet, but it sounds annoying as hell. The "undetermined" language shouldn't exist. That needs be a silent default. The user should be able select their primary language with a drop-down menu. The combo box would still exist but it'd be completely optional.
In your case it'd be nice if the "Select Language" drop-down was automatically set to the community's default language. And maybe put a little red icon next to it when your language settings are incompatible.