The lyrics are in Latin, and they've been changed to fit more to the AC's story. There's a few new parts, such as the choir part when (SPOILER)Sephiroth makes the top of Shinra Tower fall on Cloud while some parts stay quite close to the original. For example, "Veni veni venias" ("Come, come, oh come") is now "Mi fili, veni, veni veni, mi fili" ("My son, come, come, come, my son"), which is still quite similar to the original lyrics.