John Calvin on Acts 2:4

4. They began to speak He showeth that the effect did appear presently, and also to what use their tongues were to be framed and applied, But because Luke setteth down shortly after, that strangers out of divers countries did marvel, because that every one of them did hear the apostles speaking in their own tongue, some think that they spoke not in divers tongues, but that they did all understand that which was spoken in one tongue, as well as if they should hear their natural tongue. “Nativum,” native. Therefore, they think that one and the same sound of the voice was diversely distributed amongst the hearers. Another conjecture they have, because Peter made one sermon in the audience of many gathered together out of divers countries, who could not understand his speech (and language,) unless another voice should come unto their ears than that which proceeded out of his mouth. But we must first note that the disciples spoke indeed with strange tongues; otherwise the miracle had not been wrought in them, but in the hearers. So that the similitude should have been false whereof he made mention before; neither should the Spirit have been given so much to them as to others. Again, we hear how Paul giveth thanks to God, that he speaketh with divers tongues, (1 Corinthians 14:18.) Truly he challengeth to himself both the understanding, and also the use thereof. Neither did he attain to this skill by his own study and industry, but he had it by the gift of the Spirit. In the same place he affirmeth that it is an especial gift, wherewith all men are not endued. I suppose that it doth manifestly appear hereby that the apostles had the variety and understanding of tongues given them, that they might speak unto the Greeks in Greek, unto the Italians in the Italian tongue, “Latine,” in Latin. and that they might have true communication (and conference) with their hearers. Notwithstanding, I leave it indifference whether there was any second miracle wrought or no, so that the Egyptians and Elamites did understand Peter speaking in the Chaldean tongue, as if he did utter divers voices. For there be some conjectures which persuade me thus to think, and yet not so firm but that they may be refuted. For it may be that they spoke with divers tongues, as they light upon this man or that, and as occasion was offered, and as their languages were diverse. Therefore, it was a manifest miracle, when they saw them ready to speak divers languages. As touching Peter’s sermon, it might be understood of the greater part of men wheresoever they were born; for it is to be thought that many of those which came to Jerusalem were skillful in the Chaldean tongue. Again, it shall be nothing inconvenient if we say that he spoke also in other tongues. Although I will not much stand about this matter; so that this be out of doubt, that the apostles changed their speech. “Vere mutasse,” truly changed.

Acts 2: 5-12

5. And there were at Jerusalem Jews abiding, godly men, out of every nation of those which are under heaven. 6. And when this was noised abroad, the multitude came together, and was astonished: because every one heard them speak in his own tongue. 7. And they wondered all, and marveled, saying amongst themselves, Behold, are not all these which speak men of Galilee? 8. And how do we every one hear them speaking in his own language, wherein we were born? 9. Parthians, and Medes, and Elamites, and inhabiters of Mesopotamia, Judea, and Cappadocia, of Pontus and Asia, 10. And of Phrygia and Pamphylia, of Egypt, and of the parts of Libya towards Cyrene, and strangers of Rome; 11. Jews and Proselytes, Cretes and Arabians, we hear them speaking in our tongues the wonderful works of God. 12. Therefore they were all amazed, and marveled, saying one to another, What meaneth this? Others mocking, said, They are full of sweet wine. “Musto,” must, new wine.