Jónsi's first language is Icelandic.
He also speaks English, according to the official Sigur Rós web site: 
On the first three Sigur Rós albums (Von, Von Brigði, Ágætis Byrjun), Jónsi sang most songs in Icelandic but two of them ("Von" and "Olsen Olsen") were sung in 'Hopelandic'.
All of the vocals on ( ) are in Hopelandic.
Hopelandic (Vonlenska in Icelandic) is the 'invented language' in which Jónsi sings before lyrics are written to the vocals.
It is not an actual language by definition (no vocabulary, grammar, etc.),
but rather a form of gibberish vocals that fit to the music and act as another instrument.
Jónsi likens it to what singers sometimes do when they've decided on the melody, but haven't written the lyrics yet.
Many languages were considered to be used on ( ), including English, but they decided on Hopelandic.
Hopelandic (Vonlenska) got its name (from a journalist, not Jónsi himself) from the first song which Jónsi sang on, "Hope" (Von).
Jónsi's first language is Icelandic. He also speaks English, according to the official Sigur Rós web site: 
On the first three Sigur Rós albums (Von, Von Brigði, Ágætis Byrjun), Jónsi sang most songs in Icelandic but two of them ("Von" and "Olsen Olsen") were sung in 'Hopelandic'. All of the vocals on ( ) are in Hopelandic. Hopelandic (Vonlenska in Icelandic) is the 'invented language' in which Jónsi sings before lyrics are written to the vocals. It is not an actual language by definition (no vocabulary, grammar, etc.), but rather a form of gibberish vocals that fit to the music and act as another instrument. Jónsi likens it to what singers sometimes do when they've decided on the melody, but haven't written the lyrics yet. Many languages were considered to be used on ( ), including English, but they decided on Hopelandic. Hopelandic (Vonlenska) got its name (from a journalist, not Jónsi himself) from the first song which Jónsi sang on, "Hope" (Von).
Jónsi