I almost spit out my beer at "answered me back in English".
My favorite language story is from when I was studying in eastern France (Strasbourg). I went across the border to the train station in Kehl, Germany, as I reasoned that train tickets inside Germany (I was on my way to Berlin) would be vastly cheaper than trying to buy the same ticket with an origin in Strasbourg.
Anyway, I know a little bit of German, and suddenly I understood why foreign language classes always start you off with directions and tickets and timetables and so on. So I was all set with my basic train vocabulary. We went back and forth and conducted our business. I gave him my American passport at the end, he seamlessly transitioned to some pleasantry in English as we concluded, and I walked away with my train ticket.
Well, as I walked away, I was pondering his lack of surprise that I was American (which should not have been surprising; my crappy German is clearly not a Frenchman's crappy German), and I replayed the conversation in my head. As I got to the "smoking or non-smoking" part, I realized I could not recall the German word (rauchen, for those keeping score at home). Eventually it came to me, and I realized that he had actually said "smoking oder non" (oder being German for "or").. Well, of course, English with a German accent sounds like German to an English speaker...
And I suddenly realized that we had probably conducted virtually all of the transaction in English, and I had no clue. He probably switched to German-accented English as soon as I did a poor job of asking for a 2nd class ticket to Berlin arriving at this time on that date, and I didn't even realize it.