I really don't think the accent thing is a big issue - look how many other actors playing German's, Russians etc have never bothered with the accent, and just used their own.
Sean Connery in Hunt For Red October, Harrison Ford in Widowmaker...and indeed the whole cast of The Eagle Has Landed, who just spoke with perfect English accents for the whole film.
Plus, over-the-top or over-pronounced foreign accents can often ruin a movie - I am reminded of Firefox, particularly most of the English actors playing Russians in the control room when they are trying to track the stolen Mig...a large part of the dialogue was virtually unintelligible.
I think you have to allow filmakers and actors some leeway here for practical reasons - if you want to be really picky about it then the actors should learn German and speak it, and it should be subtitled. But of course that's not going to happen.
Or, they learn a German accent, which means the whole cast have to speak in a German accent. That can complicate the casting process and limit their options.
Or you do what has been done many times before and just ignore it - we know they are German, so use your imagination. It's been done before, so what's the big deal?