I have thought about this for some time now, and although you misspelled mythology, my fastidious conscience will shrug it off like Clinton's "justifiable" murder of two million children in Iraq and targeting of bomb shelters via handy-dandy-working-force-eliminating CRUISE MISSLES. ANYWAY, it was actually China that bombed the US, and even if it was Russia there'd still be the problem of different languages.
Now, this wouldn't REALLY weigh down on the plot except for the fact that one of the biggest parts of Fallout's motif is its comedic portrayal of American culture. They could make fun of Starcraft in the Korean area of the gameworld, but that's about all that gamers would really laugh at. It would have to be marketed especially to Russia and China for it to really sell well AND be well recieved. Over here, however, it just wouldn't be FALLOUT anymore. Maybe the team worked on it, but it would be a generic cut-and-paste RPG despite existence of guns. Fallout's revolutionary hair style of motif would run out of gel, and maybe even stick up in the morning and forget to water down.