To make fun of the French, you add "les" ("LAY") to everything you say, perhaps snort and laugh nasally, or say "ooh-la-la." Perhaps you limpen your wrist to further the image.
To make fun of the British, you act stuck-up and bring out your best fake British accent. Come on; everyone's got one.
To make fun of the Japanese, you change the letter "L" to "R," say "ohhhhhhh" a lot, and speak quickly with broken grammar.
We're not very good at making fun of Mexicans; usually we just name menu items at Taco Bell.
How do you make fun of Americans?
It probably differs a lot depending on where you are. The Spanish, for example, probably have a very different way of making fun of the French. I'm thinking that typical American jokes involve a hick accent and pretending to be stupid. This was probably the case even before our current president. Anybody know?
Currently listening: The Roots, Dice Raw of Nouveau Riche, and Malik B.—Here I Come