Sometimes people speak to you in a language you’re not expecting. You might assume they’re speaking in the language you were expecting, and try to make sense of their utterances as if they were in that language, which just confuses you.
An example of this happened the other day when I went to pick up my laundry from the laundrette. One of the guys who works there is Chinese, and I’d heard him speaking Mandarin on the phone so thought I’d speak to him in that language. He wasn’t expecting this and thought I was speaking English, so couldn’t understand me. I explained, in English, what I was saying, then we chatted in Chinese for a bit and he understood me perfectly.
A similar thing happened to me when I was in Dublin in June – most of the kitchen staff in the hostel I was staying in were Chinese and I thought I’d be clever and order my breakfast in Mandarin. They weren’t expecting this and could make head or tail of what I said to them.