They call it “speaking in tongues” but in fact they aren’t words from another language - they aren’t words at all. The reflects the patterns of speech of the speaker’s native language, but contains no meaning. The resemblance to human language is merely on the surface - glossolalia is only a facade of language. The syllable string doesn’t form words. It is a meaningless but phonologically structured human utterance, which the speaker may believe to be a language but it bears no systematic resemblance to any natural language, living or dead.
Celsus: “Having brandished these threats they then go on to add incomprehensible, incoherent, and utterly obscure utterances, the meaning of which no intelligent person could discover: for they are meaningless and nonsensical, and give a chance for any fool or sorcerer to take the words in whatever sense he likes”