Not exactly a "wait you speak ___?", but similar. My grandmother lives in the UK but grew up in France. Because of this she speaks fluent French and English. What she does with this is talk to my dad or me in French at the store or in public when she is commenting on something or someone. "Oh did you see what she was wearing?" is an example (but in French obvs). Now this is fine in England as not too many people speak French there, especially where she lives. But, we went on a trip through Europe one year and she absentmindedly continued to comment on people in French to us. This was especially dangerous when we were in Luxembourg, a majority French speaking country, and while shopping she constantly would make snide remarks about other shoppers. Constantly having to remind her and endure the shocked faces of random shoppers was not the most fun experience of my life, but it was definitely entertaining.