It's not a language. In linguistic studies made of various glossolalias none of them showed signs of a structure that could be called language, although they did share some common elements.
Here is one particular assessment taken from a Wikipedia article, which reflects the consensus of these studies.
Linguistics of Pentecostal glossolalia
William J. Samarin, a linguist from the University of Toronto, published a thorough assessment of Pentecostal glossolalia that became a classic work on its linguistic characteristics.[3] His assessment was based on a large sample of glossolalia recorded in public and private Christian meetings in Italy, Holland, Jamaica, Canada and the USA over the course of five years; his wide range included the Puerto Ricans of the Bronx, the Snake Handlers of the Appalachians, and Russian Molokan in Los Angeles.
Samarin found that glossolalic speech does resemble human language in some respects. The speaker uses accent, rhythm, intonation and pauses to break up the speech into distinct units. Each unit is itself made up of syllables, the syllables being formed from consonants and vowels taken from a language known to the speaker.
It is verbal behavior that consists of using a certain number of consonants and vowels[...]in a limited number of syllables that in turn are organized into larger units that are taken apart and rearranged pseudogrammatically[...]with variations in pitch, volume, speed and intensity.[4]
[Glossolalia] consists of strings of syllables, made up of sounds taken from all those that the speaker knows, put together more or less haphazardly but emerging nevertheless as word-like and sentence-like units because of realistic, language-like rhythm and melody.[5]
That the sounds are taken from the set of sounds already known to the speaker is confirmed by others: Felicitas Goodman found that the speech of glossolalists reflected the patterns of speech of the speaker's native language.[6]
Samarin found that the resemblance to human language was merely on the surface, and so concluded that glossolalia is "only a facade of language".[7] He reached this conclusion because the syllable string did not form words, the stream of speech was not internally organised, and– most importantly of all– there was no systematic relationship between units of speech and concepts. Humans use language to communicate, but glossolalia does not. Therefore he concluded that glossolalia is not "a specimen of human language because it is neither internally organized nor systematically related to the world man perceives".[7]
On the basis of his linguistic analysis, Samarin defined Pentecostal glossolalia as "meaningless but phonologically structured human utterance,
What other explanations are there for it?
In 2006, the brains of a group of individuals were scanned while they were speaking in tongues. Activity in the language centers of the brain decreased, while activity in the emotional centers of the brain increased. Activity in the area of control decreased, which corresponds with the reported experience of loss of control. There were no changes in any language areas, suggesting that glossolalia is not associated with usual language function.[10][11][12] Other brain wave studies have also found that brain activity alters in glossolalia.[13]
Learned behaviour
The material explanation arrived at by a number of studies is that glossolalia is "learned behavior".[15][19] What is taught is the ability to produce language-like speech. This is only a partial explanation, but it is a part that has withstood much testing. It is possible to train novices to produce glossolalic speech. One experiment with 60 undergraduates found that 20% succeeded after merely listening to a 60-second sample, and 70% succeeded after training:"
(source: ibid)
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