I only saw the film in the cinema when it came out, which was too long ago for me to care to do the maths, but I seem to remember the German accents were all quite off. But I might be misremembering.
It's complicated - the actors are all German(ish) and most of them are supposed to be German in-universe, including Diane Kruger's character, who's supposed to be their "in" contact, as a famous German celebrity (so whatever her accent, it's supposed to be known as authentically "hers").
The only guy who's intentionally supposed to be off is the guy who does the gesture - he's supposed to be
American British (though played by the German-Irish Michael Fassbender) and with a weird accent, but he attempts to explain that by coming from an obscure, secluded Swiss village (at the foot of Piz Palü), which sounds weird, but plausible (though IMHO and IIRC, his accent didn't sound Swiss at all, but I may be misremembering it - haven't seen it for several years, much as I actually like the movie).
If the others also sound off to a native speaker, well, that's probably beyond any possibility to be amended, because, like I said, the actors come from various parts of Germany themselves and are supposed to indeed play actual German characters (though anti-Nazi undercover ones).