Christmas carols: Psallite Unigenito
This carol is often credited to Michael Praetorius, but the music is actually an older French melody. The New Oxford Book of Carols refers to this piece as "macaronic," which sent me to the dictionary to discover that it means "mixed language," especially Latin mixed with a vernacular language (in this case, Latin and German).
I found several recordings of this, and I liked this one best:
Ensemble D.E.U.M., November 2008: