My Great Grandma was very involved in the charismatic movement, she could sometimes converse in Greek without ever having learned a word, but she couldn't understand Greek.

That's a real case of the phenomenon.

More often than not, the Pentecostals are just screaming gibberish, and it doesn't sound like genuine language, it's always something like "halshamadamanai" way too repetitive to be authentic communication. It's hard to argue that it's intentional deception, though. I think the extreme degree of crowd control, hormones pumping, etc. triggers some kind of euphoria where they just speak nonsense and they don't even realize it's just their brain short circuiting.