Funny thing happened to me, too. I'm a native German, English and Chinese speaker. I was walking down the road in Shanghai and some German girls were in front of me making jokes. I chuckled a bit because I understood what they said. They girls noticed and mistook me for a local Chinese and then began badmouthing me and saying that I was trying to be funny to gain their simpathy etc.

I decided to wait until they finished talking bad things about me. Then I answered in the most perfect and accent-free German you can imagine at how offended I was. This probably gave them the shock of their life. They kept apologizing to me for half an hour. :D

Sorry, our native languages*