We just got back from Paris, where our m.o. is always to blend in, not carry things (such as English newspapers or other nationality giveaways) and, when on the Metro, to remain completely mute for the duration of the ride. As we were standing, waiting to exit at the approaching stop, a woman who also planned to exit, pushed past and loudly said, in English, "excuse me; I must get by."
Because few people visiting Paris would miss the body language even if they didn't understand the words (in this case, true cognates of the same in English), we decided that it must be her simple amusement to guess the origins of foreigners and speak their language rather than her own, perhaps in order to feel some superiority. Her tone of voice made it clear that she wasn't participating in France's "charm offensive" in order to make visitors feel welcome....
Maybe some of this goes on in countries where visitors' native languages appear to be different -- a kind of linguistic one-upsmanship achieved by speaking the Mother Tongue to people who thought they were doing very well without translation, thank-you-very-much?
Now, when I'm having trouble speaking another language, and am appealing for help, I am SO grateful! But we are talking about a couple of very different situations.