The other day I was sitting in a jacuzzi, pretending I was in a bubbling onsen hot springs bath back in Japan. (To an onsen otaku like myself, being away from home for so long has been really hard.) There was a girl of nine years old or so sitting in the water with me, and presently her mother came over and told her, in Spanish, that it was time to get out of the water. The girl replied in rapid Spanish back to her mother, but as I listened, she peppered her speech with English phrases like, "is that okay?" or "just a little more." (I got the impression that she switched to English when she was trying to get her mother to agree to something.) In linguistics this is known as code-switching, moving from one language to another when both parties understand both languages. I do the same thing in Japanese, speaking English to my kids or other gaijin friends who understand Japanese, but switching to Japanese at random times -- for example, when mentioning numbers in Japanese, or if I want to use a specific word.
Bilinguals often switch, say, zwischen Deutsch und Japanisch.