Just before waking this morning, I had a dream during which a woman speaking to me used an Italian phrase that I myself don’t know. Doing a quick search online after I got up, I found that people often report circumstances of speaking or hearing foreign languages in dreams.
The most interesting, and authoritative, information I found is a moderated page on linguistlist.org –” Foreign languages in dreams” (1996).
Another page of comments related to the topic is on answerbag.com — “Do multilingual people think in different languages all the time?” (2006).
I’m sure there’s a lot more — will try and find more later.