"1 Now while the day of the [festival of] Pentecost was in progress they were all together at the same place, 2 and suddenly there occurred from heaven a noise just like that of a rushing stiff breeze, and it filled the whole house in which they were sitting. 3 And tongues as if of fire became visible to them and were distributed about, and one sat upon each one of them, 4 and they all became filled with holy spirit and started to speak with different tongues, just as the spirit was granting them to make utterance.
5 As it was, there were dwelling in Jerusalem Jews, reverent men, from every nation of those under heaven. 6 So, when this sound occurred, the multitude came together and were bewildered, because each one heard them speaking in his own language. 7 Indeed, they were astonished and began to wonder and say: “See here, all these who are speaking are Gal·i·le´ans, are they not? 8 And yet how is it we are hearing, each one of us, his own language in which we were born? 9 Par´thi·ans and Medes and E´lam·ites, and the inhabitants of Mes·o·po·ta´mi·a, and Ju·de´a and Cap·pa·do´ci·a, Pon´tus and the [district of] Asia, 10 and Phryg´i·a and Pam·phyl´i·a, Egypt and the parts of Lib´y·a, which is toward Cy·re´ne, and sojourners from Rome, both Jews and proselytes, 11 Cre´tans and Arabians, we hear them speaking in our tongues about the magnificent things of God.” 12 Yes, they were all astonished and were in perplexity, saying one to another: “What does this thing purport to be?” Acts 2:1-12
It happened in order that the people gathered within hearing distance could understand what was said in his/her native language. These were understandable languages, not unintelligible babbling. I invite you to look it up in your Bible translation of choice.
"1 If I speak in the tongues of men and of angels but do not have love, I have become a sounding [piece of] brass or a clashing cymbal. 2 And if I have the gift of prophesying and am acquainted with all the sacred secrets and all knowledge, and if I have all the faith so as to transplant mountains, but do not have love, I am nothing. 3 And if I give all my belongings to feed others, and if I hand over my body, that I may boast, but do not have love, I am not profited at all.
4 Love is long-suffering and kind. Love is not jealous, it does not brag, does not get puffed up, 5 does not behave indecently, does not look for its own interests, does not become provoked. It does not keep account of the injury. 6 It does not rejoice over unrighteousness, but rejoices with the truth. 7 It bears all things, believes all things, hopes all things, endures all things.
8 Love never fails. But whether there are [gifts of] prophesying, they will be done away with; whether there are tongues, they will cease; whether there is knowledge, it will be done away with. 9 For we have partial knowledge and we prophesy partially; 10 but when that which is complete arrives, that which is partial will be done away with. 11 When I was a babe, I used to speak as a babe, to think as a babe, to reason as a babe; but now that I have become a man, I have done away with the [traits] of a babe. 12 For at present we see in hazy outline by means of a metal mirror, but then it will be face to face. At present I know partially, but then I shall know accurately even as I am accurately known. 13 Now, however, there remain faith, hope, love, these three; but the greatest of these is love." 1 Corinthians chapter 13
NWT of the Holy Scriptures.