Ok, so the main argument against it being set elsewhere is that american culture is too deeply-rooted in the fallout experience to be removed and still have the same game.
But american culture has found its way everywhere. I think an outsider's perspective could be fresh and interesting, but still feel a lot like Fallout.
One idea, that maybe those of you who know about this sort of thing could fix up and improve, would be to set it in Japan.
You could have the samurai reborn! Gangs who wish to reclaim Japan as their own, travelling the wasteland with armour and katanas.
Or you could have younger gangs, binge-drinking nuka cola, wearing baseball caps and using guns to assert their power.
See what I mean? It can be all about American culture, without being in America.