I forgot all about this story, but once when I was an American serviceman in Berlin, my roommate and I wore our Air Force uniforms home on the subway.
My roommate had a German mother and American father and spoke German as well as he did English. I also spoke German pretty well. But you know how Americans tend to be seen as not speaking any other languages.
Anyway, a couple people seated across from us started talking about us as if we weren’t there — in German, of course.
They weren’t nasty, but they were saying things they definitely would not have said if they realized we understood them.
So as the train pulled into our station, we stood up and began to loudly debate whether to have pizza or doner kebab for dinner. In rapid German with Berliner slang.
The people across from us turned so red you’d have thought they were candied apples. Lol!
Just to show it never pays to assume speaking a particular language assures privacy. :-)