As the bilingual observes an anglophone listening, he or she will ask "do you understand this" in English with a probability 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 asking "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 to me to not be harrassed about it. Friends of mine.
It's not even a real question. What can you say, yes, completely, no then what am I doing or the real answer, which kills the moment. The answer to do-you-understand-this-language is always complex. It's a dumb to interject it when somebody's trying to listen.
It's a rude question in the wrong language.