I'm not sure "Stille Nacht" should be counted as "folk," since we know who wrote the music and the lyrics, and it was composed for a specific occasion, with written sheet music.   But it's been adopted by folks all over the world and translated into many languages, including several versions of "Silent Night" and several of "Noche De Paz," and "everybody knows it," so I guess it's a folk song in the same way that some of Stephen Foster's songs are.

It IS one of my favorites, and I love to sing it in German, English, and Spanish.

Genie