As you already to intend to buy Win 7 Ultimate this should offer you the possibility to change the interface language from English to German (and thirtysomething other languages) a part of the features Win 7 Ultimate offers. This was one of the reasons I bought the Ultimate version in case I might move back to Germany.
I use some German progs on Win7 UK version and have so far encountered no issues, even without changing the interface language.
Great input FeX IronC. Thanks!!