Not that accents can't be faked (and Nolan loves his British actors), but when the main contenders are two Brits and a German, is the character is especially likely to be American?
In do not care about the nationality of any actor. I really think there are much more important and essential criteria to judge whether or not a certain actor is right for a part. I have seen Brits play American and I have seen Americans play Brits before. I have seen Irish play Germans and have seen Italians play Indians and Hindus and Arabs.