This happens more often than one would think.
As you may have noticed, I am fluent in English due to my seventeen years of experience with it. No questions there. I think in english, I write in english, but I don't always speak it.
Most of my dreams are in the wrong language. My mind is somehow a language scholar, and able to speak any dialect of any known (or pretend) tongue. At least in my dreams, anyway. I have had several dreams in Spanish, Russian, Japanese, and even a few in German. Too bad I can't remember them when I wake up.
No sé lo que estoy diciendo.
Ich weiß nicht, was ich sage.
Non conosco di che cosa sto parlando.
That's the best I can do.