As the bilingual observes an anglophone listening, he or she will ask "do you understand this" in English with a probability factor that quickly approaches 100%. Few make it past the first minute.
It's one of the most stupefying behaviors in the human repertoire.
I've tried everything I can think of. I've tried, repeatedly and without any effect, saying "excuse me?" in the target language. I've tried asking "why are you asking me in English?" I've tried looking out the window and waiting for the idiot to figure out that I don't want to speak in English when I'm trying to understand what somebody is saying in another language.
Nothing that I do or say makes any difference. I think that only a demonstration of mastery would suffice.
I've been asked by people who know that it's important for me to not be harrassed about it. Friends of mine.
What's ludicrous is that there can be no simple answer, and so it's not normally even a real question. The answer is nearly always complex in varying degrees, so in most social settings it's a dumb question in the wrong language.