Lyric discussion by sokorny 

Cover art for Ultra-violent lyrics by Cavalera Conspiracy

The "gibberish" is mainly Nadsat (which is a fictional language from A Clockwork Orange, which coincidentally Sepultura released A-Lex a concept about the same story a few months later).

However, dies irae is Latin for "day of wrath".

http://soomka.com/nadsat.html