Had great fun in the market once following some people around who were speaking a European language I also speak. Although their conversation was mundane, along the lines of, "Gee, they actually have THIS, HERE" and "I can't believe anybody would eat THAT", it might have offended THAT eaters, or those who thought they were being considered rubes, and taught me well not to assume that other people around me, no matter what they look like, might also speak the language I am speaking with my companion. I entirely second the "be careful; you never know who might be listening!" posts.