As I noted yesterday, languages which I acquired as a child occupy a different part of my brain from languages I acquired as an adult. If you speak to me in a childhood-acquired language, the information goes directly into my brain via some sort of low-level connection, and I barely even recognize what language it is you're speaking. On the other hand, if you speak to me in a language I acquired as an adult, it requires a conscious effort to process the information. (I don't have to translate what you speak before I can understand it, but the act of understanding requires a bit more effort.)
I got to experience this phenomenon with my Chinese-speaking nieces. If you spent time in their home, you'll hear three languages spoken. The adults speak to each other in their local Chinese dialect, they speak with the children in the regional Chinese dialect, and of course there's English. (The adults also speak Mandarin Chinese but you don't hear it often in the house.)
Since none of the Chinese dialects I know overlap with the dialects spoken by the children, my conversations with the nieces are in English. Over time, I started learning the regional dialect, and whenever possible, I would use it when speaking with the children. (I.e., when what I wanted to say had nonzero intersection with what I knew how to say.)
But even when I spoke with the nieces in Chinese, they always responded in English.
One day, I asked one of the nieces a simple question in Chinese, something like "Do you want to drink some water?" She looked at me and said, "大姑丈講中文!" I give her sentence in the original Chinese because it is ambiguous. I interpreted it to mean, "Uncle, speak Chinese!" And I was confused, because, well, I was speaking Chinese.
It was later explained to me that my niece meant the other interpretation of the sentence, which is "Uncle is speaking Chinese!" In other words, she was expressing surprise that I was speaking Chinese. I'd been doing this for months, but this was the first time she noticed.