I have a same situation before. Few years ago, a good friend of mine merried a Shanghaiese girls from Hong Kong, both families came to Canada for their wedding. As a good friend, I drove the bride's family around.
One day, in the car, the brode's sister criticized about my friend's family in Shanghaiese. It just happened I speak that lanuage too. I just kept my mouth shut and pretended I didn't understand. After awile, the sister wanted a klneenex, and asked her cousin who sit at the front to hand her one. Naturally, I just pick a few and handed to her.
From the mirror, I saw her face were all red and asked me "oh, you understand Shanghaiese ?"
it was funny.