You'll almost certainly be fine ;) I realise that's not as definitive as you might like, but it's not something we've tested, so I don't think anyone at CA would be able to answer that question with 100% certainty.

In fact, even if you were to install say the US expansion pack over a Korean install of the main game it would probably still work, as the game uses unicode internally. The only snag might be in the font handling, and that would be a case of suck-it-and-see. Although doing such a thing would certainly be considered 'cruel and unusual punishment' by the Gods of Software.