Aa far as I know, the community does set a default. I've had some comments default to English, or blank/undefined depending on the community.

It's just not clearly indicated.

Without realizing, I tagged my last few posts as "English." If I go into my user settings and select "Undetermined" in my languages, they become invisible to me. If I select English, they become visible but now a huge number of posts on the site disappear. If I deselect ALL languages, then I can finally see most things (maybe?). It's kind of a mess and I think the safest bet is to just not select a one at all for your posts until they overhaul the languages.

e: The language stuff really either needs a complete overhaul, or needs to go entirely. Are you even going to be able to see my reply here if you've selected "English" in your settings? I guess we'll see.