That post reminded me of this hilarious story by David Sedaris about a subway ride he took in Paris. Two American tourists mistake him for a Frenchman and, thinking he can't speak English, start talking about him.
I also have a story to share. Mr. Nag and I were touring a winery in Banyuls-sur-Mer, France with a German couple and a guide who spoke both German and English. At the end of the tour the Germans purchased a few cases of wine. Then the woman turned to the guide and, referring to us, said "These people won't buy any wine. Canadians are too cheap." We don't speak much German but understood what they were saying. I'm still kicking myself because I didn't have a snappy comeback handy.