on von, agaetis, byrjun and takk, jonsi sang most songs in icelandic but a few of the songs were sung in 'hopelandic' Hopelandic is a sort of gibberish that acts as an instuments to fit the music, a bit like laaaing and dumming.
Further to the other answers, on recent hit Hoppipolla, for example, most of the lyrics are in "proper" Icelandic, apart from the very high-pitched bits near the end, which are in Hopelandic.
The lyrics to several Sigur Ros tunes (with translations) can be found here: