I agree with the above - it makes me cringe to see those old war films in which German officers conversing with each other do so in German-accented English. I genuinely don't get why you think that would be better, I think it's silly. You do mean when they speak with each other, don't you? Not when speaking to English or American characters?
One of my favourite films is Conspiracy, showing the senior figures in Nazi Germany formulating the basis of the Holocaust. It's just a bunch of men around a table, talking, and is riveting. They too adopt the approach of everyone just speaking 'normally'. If they all adopted German accents, it would be near-unwatchable for me.
The idea is that, as with many things in films, we use our imaginations to accept they are speaking German to each other. The use of a cod accent is then not necessary.
I did want to see Valkyrie when it came out but never got around to it.