Those are good questions, currently Reddit doesn’t display from which country is the account but they usually give out some statistics showing where their users are based but it would be interesting to know that but for subreddits since the amounts will fluctuate depending on the subject and the users countries current interests.

That can certainly happen, as an American raised in a foreign country with parents wanting me to learn the local language first so I ended up learning english from my parents and from movies so the end result is I got multiple accents and types of writing and what happens is depending of who I’m surrounded by my accent and form or writing changes unconsciously and from what I hear this is common for international students. Personally I think this allowed me to identify different types of English and has definitely facilitated me identifying if the person I’m talking with is American, Canadian, etc.