Re: Fallout 4 setting confirmed
If we're looking at non-American settings I would think China would be the most likely other nation. After all China and the US were the opposing sides in the nuclear exchange that flattened the world, so it stands to reason that they'd be the two most torn up. Plus being set on the other side of the planet in a world where very few people have the ability to cross an ocean opens up a lot of possibilities. China would likely have very different mutations than the US, so instead of death claws and radscorpions we may have some completely different monsters.
All that said I think it most likely they'll stay in America. There's a lot of backstory to build on that way.