Only the refrain of the song is in Latin, much like the "gloria in excelsis Deo" refrain used in Angels We Have Heard on High. The song itself is from a 16th-century poem in Scots. Here is the first verse; see the blog post for additional verses:
1. Jerusalem reioss for joy:
Jesus the sterne of most bewte
In thee is rissin, as rychtous roy,
Fro dirknes to illumyne the.
With glorius sound of angell gle
Thy prince is borne in Baithlehem
Quhilk sall thee mak of thraldome fre.