I know at least when I'm posting on kbin instances, undefined isn't really an option -- if you don't set the language, the post won't go through.
On other fediverse instances, there's actually auto-detection of language as an option, followed by auto-translation. I think the lemmy devs are up to their armpits in alligators at the moment, but I wouldn't be surprised if eventually both features come to lemmy once things calm down.