Yes, this happens to me a lot. I'm bilingual and where I live it is normal to mix two languages in one sentence. When I get used to speaking a word in the other language all the time, I forget how to say it in the other language. It gets confusing too sometimes. I guess this would happen more often if you live in a different country that uses a different language than your native language. For example, I have friends who moved to generally English speaking countries. Now they talk to me in English more when before they would talk to me in our native tongue. When they come to visit, they seem to have a hard time articulating what they want to say in our native language.