There IS global travel.
Dashwood's Terminal says he wasn't surprised when Moriarty got his own bar. He's been trying to get rich since he 'ended up in this country, as a baby.'
This Country, after the bombs fell America could be divided into many countries. Republic of Dev ring any bells?
A Fallout 3 loading screen says that 'Allistair Tenpenny came from England to make his fortune" or something along those lines.
Not sure about that, but England could be New England.
Then there's Dukov and his accent. As for Desmond he was probably spying in America when the war hit.
Dukov could have come from an area with alot of Russian pre-war people settled. Desmond came before the Great War not after.
People can have foreign accents but having never come from there. There's a man in FO1 with an English accent and he got that from the way people read a Robin Hood book to him guessing he learned that women love foreign accents.
Like any city there are areas like "China Town", "Greek Town" and so on. Accents from pre-war communities could have just stuck. I am sure the Shi in Fallout Two would have had chinese accents but they came just after the bombs fell because their sub was busted.
There are even medical cases where people suffered head injury and started speaking in a foreign accent.
My point is World wide travel would be rare at best. If someone came from another nation after the great war, I want to hear a kick-ass story not just speculation. People do lie you know.