John Calvin Commentary Acts 2:4

"And they were all filled with the Holy Spirit, and began to speak with other tongues, as the Spirit gave them utterance." — Acts 2:4 (ASV)

They began to speak. He shows that the effect appeared immediately, and also for what purpose their tongues were to be formed and used. But because Luke records shortly after that foreigners from various countries marveled, because each of them heard the apostles speaking in their own tongue, some think that they did not speak in various tongues, but that they all understood what was spoken in one tongue, just as if they were hearing their native language.

Therefore, they think that one and the same sound of the voice was variously distributed among the hearers. They have another conjecture: because Peter delivered one sermon to an audience of many gathered from various countries, who could not understand his speech (and language) unless a different voice reached their ears than the one that proceeded from his mouth.

But we must first note that the disciples indeed spoke in foreign tongues; otherwise, the miracle would not have been worked in them, but in the hearers. Thus, the comparison he mentioned before would have been false, nor would the Spirit have been given to them as much as to others.

Again, we hear how Paul gives thanks to God that he speaks in various tongues (1 Corinthians 14:18). Truly, he claims for himself both the understanding and also the use of them. Nor did he attain this skill by his own study and effort, but he had it by the gift of the Spirit.

In the same place, he affirms that it is a special gift, with which not all people are endowed. I suppose it is thereby clearly evident that the apostles were given the variety and understanding of tongues so that they could speak to the Greeks in Greek, to the Italians in Italian, and so that they might have true communication (and discussion) with their hearers.

Nevertheless, I leave it as a matter of indifference whether or not any second miracle was performed, such that the Egyptians and Elamites understood Peter speaking in the Chaldean tongue as if he uttered various voices. For there are some conjectures that persuade me to think this, and yet they are not so firm that they cannot be refuted.

For it may be that they spoke in various tongues as they encountered one person or another, as the occasion arose, and as their languages were different. Therefore, it was an obvious miracle when they saw them prepared to speak various languages.

Regarding Peter’s sermon, it might have been understood by the greater part of the people, wherever they were born, for it is to be supposed that many of those who came to Jerusalem were skilled in the Chaldean tongue. Again, it will be entirely plausible if we say that he also spoke in other tongues.

Although I will not insist much on this matter, provided that this is beyond doubt: that the apostles changed their speech.