Like others recently, I've been extremely impressed by LLM's ability to play GeoGuessr, or, more generally, to geo-locate random snapshots that you give them, with what seem (to me) to be almost no context clues. (I gave ChatGPT loads of holiday snapshots, screenshotted to remove metadata, and it did amazingly.) I assume that, with enough training, we could get similarly accurate guesses of a person's linguistic hist…
We actually did something like this for non-native English speakers a few months back. Check out https://accentoracle.com (most mind-blowing if you're a non native English speaker)
Edit: Tried it a few times and also got English as an accent. Pretty fun application!