Google is your friend, call in from time to time

Anyways...It's in Latin. I think this is right, theres quite a few translations doing the rounds.

Latin

Estuans interius,
Ira vehementi,
Estuans interius,
Ira vehementi,
Sephiroth!
Sephiroth!

Estuans interius,
Ira vehementi,
Estuans interius,
Ira vehementi,
Sephiroth!
Sephiroth!

Sors immanis
Et inanis,
Sors imnais,
Et inanis.

Estuans interius,
Ira vehementi,
Estuans interius,
Ira vehementi,
Sephiroth!
Sephiroth!

Veni, Veni, venias!
Ne me mori facias!
Veni, Veni, venias!
Ne me mori facias!

Veni, Veni, venias!
Ne me mori facias!
Veni, Veni, venias!
Ne me mori facias!

Karyu no hanay kata!
Gloriosa!
Generosa!
Gloriosa!
Generosa!

Sephiroth!
Sephiroth!

English Translation

Burning inside
with violent anger.
Burning inside
with violent anger.
Sephiroth!
Sephiroth!

At the moment of creation
At the moment of creation

Burning inside
with violent anger.
Burning inside
with violent anger.
Sephiroth!
Sephiroth!

At the moment of creation
At the moment of creation

Fate, monstrous
and Empty
Fate, monstrous
and Empty

Burning inside
with violent anger.
Burning inside
with violent anger.
Sephiroth!
Sephiroth!

At the moment of creation
At the moment of creation

Come, come, O come,
do not let me die
Come, come, O come,
do not let me die

Come, come, O come,
do not let me die
Come, come, O come,
do not let me die

Glorious
Noble
Glorious
Noble

Glorious
Noble
Glorious
Noble

Sephiroth!
Sephiroth!

Not sure, I think it was just a good idea to them I guess. Possible Apocolyptic battle, great god like creature....fuck it let's do it in latin.

What I do know though is that the lyrics are taken from Medieval Poetry.

That's correct, they are taken from sections of the Carmina Burana cantat by Carl Orff. The same cantata that the lyrics for O Fortuna were taken from

Here is the Advent Children Version, written by Tetsuya Nomura himself-

Latin

Noli manere, manere in memoria
Noli manere, manere in memoria
Sephiroth, Sephiroth

Saevam iram, iram et dolorem
Saevam iram, iram et dolorem
Sephiroth, Sephiroth

Ferum terribile, terribile fatum
Noli manere, manare in memoria
Noli manere, manare in memoria
Sephiroth, Sephiroth

Veni, mi fili. Veni, mi fili
Hic veni, da mihi mortem iterum
Veni, mi fili. Veni, mi fili
Hic veni, da mihi...

Noli manere in memoria
Saevam iram et dolorem
Ferum terribile fatum
Ille iterum veniet

Mi fili, veni, veni, veni, mi fili
Mi fili, veni, veni, veni, mi fili
Mi fili, veni, veni, veni, mi fili
Mi fili, veni, veni, veni, mi fili

Mi fili, veni, veni, veni, mi fili - Bass/Tenor
(Qui mortem invitavis) - Alto/Soprano
Mi fili, veni, veni, veni, mi fili
(Poena funesta natus)
Mi fili, veni, veni, veni, mi fili
(Noli nomen vocare)
Mi fili, veni, veni, veni, mi fili
(Ille iterum veniet)

Sephiroth, Sephiroth
Sephiroth

</SPAN>English translation

Won't remain, remain in memory
Won't remain, remain in memory
Sephiroth, Sephiroth
Raging anger, anger and misery
Raging anger, anger and misery
Sephiroth, Sephiroth
Fierce terror, fierce fate
Won't remain, remain in memory
Won't remain, remain in memory
Sephiroth, Sephiroth
Come, my son. Come, my son
Come here, give me death once more
Come, my son. Come, my son
Come here, give me...

Won't remain in memory
Raging anger and misery
Fierce, terrible fate
The second advent

My son, come, come, come, my son
My son, come, come, come, my son
My son, come, come, come, my son
My son, come, come, come, my son
My son, come, come, come, my son - - Bass/Tenor
(By death's invitation) - - Alto/Soprano
My son, come, come, come, my son
(Painful tainted birth)
My son, come, come, come, my son
(Won't call the name)
My son, come, come, come, my son
(The second advent)
Sephiroth, Sephiroth
Sephiroth

Actually I could imagine a Japanese version working, though it might not have the gravitas of the Latin words.

I'm actually surprised that Tetsuya Nomura took the time to create a Latin set of lyrics for AC's One Winged Angel. Clever clogs that guy!