Try speaking an obscure language they will not understand back at THEM!
They get furious, fast.
In fact they get so mad they break into English in order to complain in a way you can understand.
Samoan works great, but Italian or Gaelic will do in a pinch.
No need to be fluent, just a few words will do the trick.
If an employee got mad at a customer for that, they should be fired on the spot.