What's more mind-boggling is they all take the same form of currency.
Actually they don't. In the Revan novel it is made clear that the Empire uses its own currency. I think the game stayed true to this in the initial months when you couldn't directly trade with the Republic (if we overlook the Nar Shaddaa GTN).
You have lots and lots of ancient Greeks and Troyjans running around. Yet they all speak perfect modern english.
I hate to correct you but, apart from Peter O'Toole and few other British actors, they don't speak perfect English.