In my opinon - Gibberish. There is going to take a lot of evidance to convence me other wise.
Here is just one of the verse where people get it the Idea:
And he said to them,“Go into all the world and proclaim the gospel to the whole creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned. And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover.” —Mark 16:15-18
Once a friend of mine was told by his parents that he can not eat until he speaks in tongues and the clever git said "e-pooty-ah-tay" which was a gibberish form of "Eat pudding all day!" and so he got to eat. Parent were none the wiser.
I've never witnessed what you are speaking of and I've been to many Pentecostal and tongue-speaking churches; it certainly has nothing to do with body language. From my vantage point, it just seemed that the general assembly took it a face-value that whomever was speaking in tongues was actually speaking in tongues. No one is analyzing motives or authenticity... because that would put a lot under the microscope they don't want to be examined. I think most who "speak in tongues" genuinely believe they are, so of course their body language wouldn't betray the fact that they've tricked their self into believing. Just because they're sincere, however, does not mean anything special is really happening.
Are you a Christian, or former Christian? I think everyone has different experiences with the many facets of Christianity. I'm not personally familiar with anything you've mentioned.
I know it sounds complicated... a person really believing they're speaking in tongues when they're not. I'm not being poetical or metaphorical.
I went to church one Sunday long ago and decided to be "Baptized in the Spirit" since the pastor was preaching about it. This particular church is a Four Square church (with Pentecostal roots) and they have members who will lay hands on you and pray that you will be blessed with the "gift" of speaking in tongues.
I'll preface this by saying I'm a natural skeptic. However, I really, truly wanted to speak in tongues because I did believe it was real at the time. A whole group of girls gathered around me and began praying over me, all of them switching between English and what they believed was their Prayer Language. One girl kept encouraging me to say whatever came; just to "let it flow".
Now, I knew then, as I know now, that I could've begun speaking gibberish. When I was little, I would pretend to speak German or Chinese by imitating the sounds as best I could. The noises these girls made were no more impressive than "Sha nanana heek-tah-rah nanana!" They could make noises that sounded phonetically like words from another language... but they ALWAYS repeat the same noises over and over again, every single time they pray "in the Spirit". Most people I know, however, are not nearly as skeptical as I am. They believe those noises they're making are Heaven-inspired and they don't question that idea for one second.
I never could do it... or rather, I could never do it and believe I was doing it. I assure you, I made noises that sounded like something (once after trying to meditate and something just flowed out of my mouth that genuinely sounded like some unknown language). But I've always been skeptical, and my religious friends are/were not. They sincerely believe they're speaking in tongues, and I sincerely believe they're only making the noises I made as I kid. I don't think they're LYING to their self exactly, but rather being easily led.