Well yesterday I decided to see what I could find out on my own so I wrote an email to unofficial website called johnpowellcomposer.com where they have an "Ask a question" part. The guy was really nice and responded already with:
**
In an interview with John Powell he states:
"They [the lyrics] are the Requiem Mass, so it was just various lines
that
were appropriate to the scenes. I just dug through a transcription of
the
mass - I think the one I used was used in the "War Requiem" by Benjamin
Britten, and just looked for appropriate lines, in particular there's a
lot
of stuff with Jean Grey - there's a lot of appropriate Latin. Day of
Judgment, that kind of stuff."
***
(He didn't give a link to the interview so I don't know where it came from or if it has been posted already, but I didn't respond asking for link cuz I felt it was nice enough he answered the question esp. so fast.)
So then I looked up those lyrics and they are at here:
http://www.its.caltech.edu/~tan/Britten/reqtext.html
Well if you ask me I think there are parts from II. Dies Irae and VI. Libera me, but I'm not sure I will listen to it and try to compare it to the lyrics later!!