Oh my god I was just thinking about this subject! I was thinking, like, maybe the nations have over time made up some sort of pidgin language among themselves as a lingua franca, or maybe the nations from the same language family speak some older form of their shared language with each other. (Like, IDK, the Romance nations speaking Latin.) Is it really obvious I study linguistics?

But seriously, this prompt. It's amazing. I might write something if inspiration strikes, but definitely seconded.