I lived in Germany for a little over 3 years when I was in the Army. I ate at a lot of restaurants and interacted with the locals quite a bit and I never heard/saw them eating mayo on beans. They do love to put it on their fried taters though. Either one is disgusting to me.
But I would not put it past them to come up with something completely gross like mayo on beans because one of the girls I dated over there showed me what a Glass Bottom Boat was and this was 15+ years before Urban Dictionary was even thought of..................