Why didn't Burton and Eastwood adopt German accents?

Clark Gable always advised younger actors he worked with to never use accents if they were playing someone other than a contemporary American. His thinking was the audience is willing to believe you as a Russian, or Austrian, or Eskimo because that's what the script says you are. But if you start trying fake accents, it will take the audience out of the story, like "Why is Robert Taylor trying to sound like Akim Tamiroff?" Gable tried to use an Irish accent in PARNELL, and it was a box-office dud.
"May I bone your kipper, Mademoiselle?"

Much as I enjoyed this movie, it is pure hokum and this is just one example. Yes, they are meant to be fluent in German so one just assumes that really they are talking German while among the Nazis, but if you came into the film late, the whole thing appears absurd. It reminds me of a pre-war movie, Night Train to Berlin I think, where two English bods overhear a German plan by eavesdropping, but the Germans are conveniently speaking in English!

It is a coincidence in this film (or maybe not, I don't know) that all the ones chosen for the mission who mostly are meant to be traitors also happen to be fluent in German or the mission could not go ahead. The whole thing is ludicrous, I mean if you are going to set up a thing like that, why start by having them infiltrate the almost inaccessible fortress, why not pick something easier? Or have I missed something among the double cross plot? Possibly. And of course, it seems only Burton knows what is going on, so if he got shot early on, they would all be proceeding on a fool's errand mission not even knowing the real background of it, as Eastwood (presumably) is out of the picture.