Because there are numerous example of people claiming to speak in tongues- record them and do some basic research to see if they are speaking a real language.
I absolutely hate deception in the name of religion. And some of the worst liars I ever met were the biggest bible thumpers.
My next door neighbor when I was growing up attending every mass every week and her son grew up to be a pastor. She was the meanest nastiest withc you ever met (out of church) and I think she killed my dog (seriously, I think she poisoned it)
Not one linguist said that glossolalia was a foreign language, Yet may I share just one letter out of many. A letter from Bruce R. Gordon, Head of the Department of Linguistics and Foreign Languages, University of Alaska at Fairbanks, says that he remains skeptical, not only because of the linguistic material, but he also verified two cases of speaking in actual foreign languages which he proved fraudulent beyond any question, Bruce goes on to say, "In the one case, despite denials, the person involved had lived in Africa and had had direct contact with several African tongues."14 In all the chances the tongue-speaker has had to convince the language expert he has failed to do so.and finally Speaking in Tongues Linguistic Study which is quite controversial.