I have the Thai language installed on my computers. It is my wife's native language, and she visits Thai sites. Obviously, I keep English as the default, and hardly ever switch, but the character set is there.
Oddly, when I go to log in to Facebook, the login page comes up completely in Thai. No other site does that (other than the ones from Thailand!). I click the "login" button, and Facebook comes up in English, although the Thai characters show (of course) in some posts by my Thai friends.
I also notice that the Facebook "Like" button (and text) here shows up in Thai.
Why is that? I realize that it has nothing to do with Threads, but I know there are a lot of smart people here. I'm sure there are others here with multiple languages running.