German here. Everyone I know puts ketchup on their hotdogs. Ketchup and mustard. =D We also do the fries with mayonnaise thing, lol
When it comes to food I had a lot of Americans and Canadians tell me how weird it is, that we don't put our eggs in the fridge. Even in the store, they are not cooled. It's totally normal here, but it seems that it blew a lot of tourist minds. 
I also had a french guy telling me that I drank my Absinthe wrong. Totally bonkers. He said you're not supposed to set a sugar cube on fire for that. But ... that's the fun part!