Some years ago, I went out to dinner with a group of friends to a Chinese restaurant, and when the check was delivered to the table, one of my friends looked at it and handed it to me. "It appears to be written in some sort of secret code."
It was written in Chinese.
I pointed out that they probably chose the worst possible code in the world, seeing as they chose something known to over a billion people.
Using a foreign language as a secret code is common when walking around out in public. You can say whatever you want about the people around you, and you can have a certain degree of confidence that your secret code will not be broken if you choose a language sufficiently obscure relative to the situation.
A colleague of mine told me that in his college days, he went on a trip to Germany with a few friends, one of whom spoke German. (I'm led to believe that knowing German comes in handy in Germany.) They boarded a train and introduced themselves to a pair of German students who shared the cabin with them. The German students spoke English, so there was some opportunity for small talk, but eventually the conversation petered out and the two groups conversed among themselves.
The German students started talking to each other about their cabinmates, saying things that were not entirely complimentary. The German-speaking member of the other group leaned over to another member of his group and announced in a stage whisper, "They think we don't understand German."
The German students promptly shut up.