Vonlenska is a term used to describe the unintelligible lyrics sung by the band, in particular by Jón Þór Birgisson. It is also commonly known by the English translation of its name, Hopelandic. It takes its name from "Von", a song on Sigur Rós's debut album Von where it was first used.

Vonlenska is a non-literal language, without fixed syntax, and differs from constructed languages that can be used for communication. It focuses entirely on the sounds of language; lacking grammar, meaning, and even distinct words. Instead, it consists of emotive non-lexical vocables and phonemes;  in effect, Vonlenska uses the melodic and rhythmic elements of singing without the conceptual content of language. In this way, it is similar to the use of scat singing in vocal jazz. The band's website describes it as "a form of gibberish vocals that fits to the music"; it is similar in concept to the 'nonsense' language often used by the Cocteau Twins singer Elizabeth Fraser in the 1980s and 1990s. Most of the syllable strings sung by Jón Þór Birgisson are repeated many times throughout each song, and in the case of ( ), throughout the whole album.