WTF = False Positive
As someone who has done a fair bit of living abroad in Foreign Language Land, I think it is quite possible that the scenario went askew simply because of a moment of confusion that had nothing to do with language. I've been there many a time myself, where my wondering things like "why the heck is he asking ME about THAT" causes people to automatically switch languages, even though language was never the problem. Because a look of confusion, regardless of the reason, creates false positives in the "Do they understand me?" test.
So I think it started more like this...
Brundlefly approaches Su, microphone in hand.
Su has a WTF moment, wondering why he has picked her out and what questions he will have.
Brundlefly sees Su's confused expression and assumes it is language that is the problem.
"Excuse me ma'am, do you speak English?"...