When you hear people speaking other languages in public, do you ever try to figure out what they're speaking? I don't think this can really be considered eavesdropping if you can't understand them (unless you can, in which case, good work). There are some I can ID a mile off (French, Spanish, Chinese) and some that take more effort. I frequently hear Korean in my part of town, for example, so I can recognize it even if I don't understand a word of it. Sometimes if I'm really lost I might ask what it is they're speaking, which usually elicits a weird look and "why do you want to know?" to which I usually explain that I'm just curious.

For example, the other day I saw a man whom I judged to be Pakistani and his companion (a white woman) on the bus conversing in English. I didn't pay attention to what they were talking about until I heard her say, out of the blue: "Yeah, I'm learning Urdu" followed by something containing the word acchā, to which I responded: "Oh yeah, اردو اچھی زبان ہے." I find people are much more forgiving about cutting into their conversation if it's in a different language.

I find that people are sometimes more receptive if you make a good guess. That is, "Is that Hungarian?" might net you fewer cold stares than, "What is that you're speaking?" And if you nail it, people are usually quite surprised (and sometimes appalled, as they replay the last several minutes of conversation in order to see whether there was anything very personal or offensive in it!).

yes, i have experienced this as well