Comment by zo1
I once had my nationality "leak" just by the specific addition I chose when ordering a specific hamburger-like dish at a food-stall. Something akin to choosing to put mustard on a hot-dog kind of situation. The person looked at me, knew, got confused as I had a perfectly neutral and non-native accent, and then just had to ask to confirm their suspicion.
Mayo with fries would be a German likeness. Very odd in the States.