There is no indication of that besides prose you are taking as a fact.
You are so high struck the world is destroy because the US was destroyed? let me tell you one thing ... civilization did fine for well over 5000 years before North America was re-discovered by the Europeans.
I dont see the reason to why the rest of the world would be destroyed, it would make no sense for the US to launch its nuclear arsenal to anyone else besides China and the same goes with China ... only because the intro makes prosaic statements does not mean they are facts.
Nice passive aggressive america hate bro.
So here's the thing, anywhere not hit by a nuke would receive fallout (of the real kind) than weather changes and other effects brought on, so people would survive, but they'd would've been through hard times the whole "World Bathed in Fire" idea is probably romanticized, it doesn't matter if it was fact then, we have statements from bethesda staff saying there's society in europe.
A major reason america did so well was vaults, we know there were these across america (including alaska, hawaii, and annexed canada) and we don't know whether other countries had other vaults.
So the place to start with a game in annexed hawaii, canada or alaska (please Alaska, with DLC here in the southeast) and maybe hint to nearby countries and progress to that country, somewhere on the rim, to ease fans into the foreign fallout feel then see if it's something we'd all be up for.
But it's they're property and they'll do whatever they want, so bickering about whether or not it works is pointless,
BETHESDA SAYS THERE IS, SO THERE IS.