My Japanese father-in-law is studying Chinese. He's a major Sinophile, and I'm pretty sure that when he retires we'll be able to find him wandering around some obscure places in China. He's probably pretty happy that his daughter and son-in-law speak Chinese, and that our kids are effectively being raised trilingually, with Chinese one of the three.
I'm now back in Japan after a two-week stint in China, and I've come back with a suitcase full of Chinese children's books for the kids. When my father-in-law heard this, he wanted to see them, and he immediately grabbed one and started going through it, asking me questions about what they were saying.
He was asking questions mostly in Japanese but with some of his Chinese thrown in there. I found that, if he used Chinese, in replying to him I'd slip right into Chinese, even though I knew I should be using Japanese for him to understand. He might say something like 这是什么？ ("What is this?"), and, knowing full well that if I just broke out in Chinese to explain it he probably wouldn't understand, my first reaction was nevertheless still to start off with Chinese. It was as if by hearing the Chinese my mind had switched into Chinese mode and I had to think consciously to switch it to Japanese.
I found I was having particular trouble when I had to explain a Chinese phrase in the midst of Japanese. One book he looked at is called 丹利的菜园 ("Tanley's Vegetable Garden"). (Tanley is my best guess as to what the name is supposed to be in English.) While Japanese have an edge in understanding words made of Chinese characters based on meaning, such as 菜园 ("vegetable garden"), phonetic words, such as 丹利 (Tanley), don't mean very much to them. So he asked me what that was. To answer him, I said, "I think the rabbit's name is Tanley," using the Chinese word for Tanley: ウサギの名前は丹利だと思う. To say that, I had to be conscious of what I was saying in order to avoid continuing on in Chinese.
More broadly speaking, being in China for two weeks seems to have put me in Chinese mode. When I got back to Japan on Sunday, I found myself responding with hèi 嘿, when someone called me, which would roughly be the equivalent of "Yeah?" in English. The Japanese equivalent would be hai, but Chinese was just jumping out of my mouth.
One of my most interesting examples of this sort of issue occurred when I just returned to the States from Brazil. In Brazil, I wasn't using anything but Portuguese, so I was completely converted over to Portuguese mode. It was maybe the first or second day I had been back. My mom was pulling something out of the dryer and asked me a question about the clothes. I responded to her, and she laughed awkwardly. The problem was that there was nothing funny about what I said—it was just some mundane comment about the clothes—but when I thought back to what had just came out of my mouth, I realized it was Portuguese. The thing that made this so interesting was that I didn't even realize that I had used Portuguese until her response made me replay it in my head.
So, I'm curious... have any of you had anything like this happen to you before? If so, drop a line in the comments.