In the european client of the game (if there is any difference at all between clients) all languages are preinstalled. I know its probably a lot of work to implement a custom installer, but in the long run would help to keep installation size in check. That also would be an idea for old expansions or locations, who aren't in "use" at the moment. Also it would be fine to change the voices independently from the written language. The german dub isn't that bad, but not that great either.