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.