In Dulci Iubilo

"In Dulci Iubilo" began in the 14th century as a macaronic song, mixing German together with Latin. The composition is attributed to the German mystic Heinrich Suso who is supposed to have had a vision of angels and to have heard them singing. He joined in the dance of the angels and then recorded the experience in this mixed German and Latin song. There is also a Wikipedia article about the carol itself.

This German-Latin song was extremely popular, and inspired an English-Latin macaronic version. You can find the English-Latin macaronic version with a MIDI file and sheet music at the Hymns and Carols of Christmas website, along with a later adaptation into English, Good Christian Men, Rejoice.

On the Internet I also found a translation of the song completely into Latin, attributed to George Litzel:

In dulci iubilo
cantate domino!
Nostri cordis gaudium
est in praesaepio
et fulget ut lux solis
in matris gremio.
Alpha est et O! Alpha est et O!

O Iesu parvule,
requiro solum te,
meumque sis solamen,
O puer optime!
commune per levamen,
O princeps gloriae!
Trahe me post te! Trahe me post te!

O Patris caritas!
O Nati lenitas!
Nos omnes cadebamus
per nostra crimina;
per hunc recuperamus
caelorum gaudia.
Simus hic eia! Simus hic eia!

Sunt ubi gaudia?
Non usquam qualia
ad angelos, canentes
Iucunda cantica,
et cymbalis ludentes
in Regis curia.
Simus hic eia! Simus hic eia!

(about this image of Alpha and Omega)