Missed out on this thread, since it said only for french S2TW players, but I also want to confirm that you can choose any of the supported languages you like!

That's one of the changes that come with Steam that I really like. Before I had to struggle to obtain the proper Addons for Rome or Medieval Total War to mix with my english version that was imported. With Steam you can select any language you like just like quadalpha described. So nothing to keep you from playing it in english, which I currently do even though I have the German version of the box sitting on my desk.