Acts 2:1-13
2 When the Day of Pentecost had fully come, they were all with one accord in one place. 2 And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting. 3 Then there appeared to them divided tongues, as of fire, and one sat upon each of them. 4 And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance.
The Crowd’s Response
5 And there were dwelling in Jerusalem Jews, devout men, from every nation under heaven. 6 And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language. 7 Then they were all amazed and marveled, saying to one another, “Look, are not all these who speak Galileans? 8 And how is it that we hear, each in our own language in which we were born? 9 Parthians and Medes and Elamites, those dwelling in Mesopotamia, Judea and Cappadocia, Pontus and Asia, 10 Phrygia and Pamphylia, Egypt and the parts of Libya adjoining Cyrene, visitors from Rome, both Jews and proselytes, 11 Cretans and Arabs—we hear them speaking in our own tongues the wonderful works of God.” 12 So they were all amazed and perplexed, saying to one another, “Whatever could this mean?”
13 Others mocking said, “They are full of new wine.”
Verse 8 says that they each heard in their own language. Each one, individually, heard every word spoken in their own language. Now, there were 120 believers in the room at that point, plus the multitude who were there who were experiencing the miracle of hearing in their own native tongues. The thing is that each person individually heard it in their own language, I believe, from every person who was speaking in tongues. What I mean is that if I, speaking English, was there and another speaking Swahili was there, and we went up to Peter and listen to him speak, I would hear it in English whereas the other person would hear the same thing in Swahili. This was not a miracle of speaking a language that they didn't know as much as it was a miracle of the hearers hearing the person in their own native tongue.
They came there curious and open, whereas the mockers came unbelieving, with the purpose of finding stuff to use against them. This is why they would only hear tongues as indiscernible nonsense. God did not grant them the miracle of hearing the tongues in their own language.
J. Rodman Williams in Renewal Theology: Systematic Theology from a Charismatic Perspective puts it like this, "...One may advance the argument that Pentecostal tongues must have been foreign language because according to Acts 2:6 "each on heard them speaking in his own language," and 2:11, "we hear them in our own tongues speaking of the mighty deeds of God" (NASB). What is said in these passages, however is not the hearing OF one's own language but the hearing IN one's own language. Such being the case, at the same moment that "other tongues" were spoken through the Holy Spirit, they were immediately translated by the same Holy Spirit into the many languages of the multitude. Closely related is the gift of interpretation that Paul describes in 1 Corinthians (12:10, 30; 14:5, 13). This gift follows a tongue and is given so that hearers can understand in their own language. In any event the tongues spoken at Pentecost and thereafter were not foreign languages but pneumatic speech-the speaking by the Holy Spirit through the mouths of human beings." He says in the footnotes, "Hence there is both a miracle of speech-other, different, spiritual tongues-and a miracle of understanding: each made possible by the Holy Spirit." He is also stating that the Holy Spirit acted as his own interpreter. I kind of disagree and say that the Holy Spirit granted each one who heard in their own language the gift of interpretation for that moment, but that's really splitting hairs.
I will also add that there was a seemingly large multitude there at that moment on the day of Pentecost. This wasn't a time as we have now where if you disagree with someone you can expect nothing more than maybe a block on Facebook or a laugh react for your dissenting opinion. This was a time where if you had the nerve to disagree with someone, you could likely have your teeth knocked out. In a huge crowd, this would be almost certain and could easily result in death. For the few dissenters to sit there and say in the midst of the crowd, "Oh! They are only drunk! Listen to them babbling nonsense!" would be very dangerous for themselves, I have a hard time believing that they would say something like that unless they were absolutely convinced and had proof right there that they were right. Not even the Pharisees did so for fear of starting a riot until they heard Jesus claim that He, as the Son of Man, would be sitting at the right hand of God and coming on the clouds of Heaven in Matthew 26. Only then did they publicly accuse Him. I have a hard time believing that the dissenters would be so bold to do that in the midst of a crowd that disagrees with them unless they were literally hearing nothing but nonsense because the Spirit didn't open their ears to hear.