I've frequently heard people say that they think it's rude when others speak a foreign language in their presence. I admit that when I was much younger I might have agreed, just as I was embarrassed and annoyed when my parents would speak Taiwanese to me in front of my friends. But now that I'm a mature, multilingual adult, I'm baffled by others who continue to feel this way.
The most common explanation I hear for such resentment goes something like, "I don't understand what they're talking about...are they talking about me, or what?" To this I would respond: Speaking a foreign language is not like whispering, which I do agree is rude. In my experience, when people speak a foreign language to each other it's because it makes more sense to do so for reasons of fluency or understanding, not because they are purposely trying to exclude others. To assume that the parties are talking about you is, in my opinion, paranoid and egocentric. I speak Taiwanese to my parents all the time because it's my only opportunity to practice the language and they tend to understand me better when I do. Why should I switch to English when others are in the room simply to facilitate their eavesdropping?
The most recent incident in which an acquaintance described language-related rudeness was at a nail salon; during her pedicure, the woman working on her feet completely ignored her and instead spent the entire half hour chatting in Vietnamese with her colleagues. Now, I agree that this behavior was rude, but I would assert that the rudeness was because the customer was being ignored, not because there was a conversation held in a foreign language. I've been waited on in stores by teenagers who don't even look me in the eye as they're ringing me up, chatting with their coworkers in English. It's the unprofessionalism that's the issue, not the language.
There are, of course, instances in which the use of a foreign language is inappropriate. In my group at work, for example, over half the engineers are from China and Taiwan, and sometimes I feel that there is more business conducted in Mandarin than in English. While I don't make a habit of eavesdropping on every conversation that takes place in someone's cubicle or in the hallway, there have been numerous occasions in which my limited Mandarin allows me to detect that I should really be part of an ongoing discussion and I join it; usually my colleagues will recognize that my presence is valuable and immediately switch to English. That's the way a lot of research-based work is done; a couple of people start talking about something, then someone overhears it and walks over to give his input, and eventually a useful exchange is had. My German and Korean coworkers, however, who don't speak Mandarin, have no such opportunity, and I believe this is ultimately detrimental to the company.
This kind of situation is unique, though, and most objections I hear to the use of foreign language have to do with personal matters. I certainly don't care at all if my coworkers talk to their wives in Mandarin on the phone. And I can't help but wonder if there is an element of racism in such objections; I hear them most frequently against Latinos and Asians, whereas I've never heard anyone complain about people speaking, say, German or French. Is the perceived threat more about the existence of these other races, and the communication habits that make their presence obvious, than it is about the language itself? It's worth some more thought and examination.