Fairly new too - why wouldn't you be able to answer if the post is set to 'Undetermined'. Haven't had any issues yet.

Unless someone goes in and changes their shown languages to English in their settings, threads with the default of English won't show. It even warns of that if you attempt to change languages in your profile.

So leaving it undertermined actually means more people will see it.

My issue was that Jerboa for Lemmy doesn't show the "Language" selection. I'll probably have to find another Lemmy client.

I just select English in the language setting before posting. It is at the bottom before submit when creating a post. The magazine or whatever has to be created with the language.