Q:   On the day of Pentecost, in which language did the apostle Peter speak when he preached to the many Jews who spoke in other languages?

A:   In Acts 2:4, the record states, "And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit was giving them utterance." First, note that the Holy Spirit fell on every person in the room. This included the eleven apostles along with Matthias, one of their number now making twelve in all (Acts 1:26). Further, it states that each one of them began to speak in other tongues by the Spirit's power.

Concerning the tongues, these were simply known languages, many of which are listed in verses 8 through 10. More can be learned about the use and purpose of the gift of tongues in 1 Corinthians. There were a total of nine spiritual gifts, listed in 1 Corinthians 12:8-10. These gifts were for the "common good" (verse 7). Concerning tongues particularly, Paul had much to say in 1 Corinthians 14:1-19. In short, the gift of language was to be used only as a teaching tool. The ability to accurately speak in any language that the need of the moment may require allowed the early disciples to quickly focus on preaching the gospel instead of learning foreign languages.

This is exactly what happened in Acts 2. When the noise from heaven in the house (verse 2) was heard in the street, the Jews who had come from all around for the feast day apparently gathered to the house (verse 6). The apostles took advantage of the situation and used their gift to begin immediately teaching those people in their own individual languages. With 12 men, each "speaking of the mighty deeds of God" (verse 11) several different teaching groups could quickly have been formed in different languages, the Cyrenians in one group, the Egyptians in another, and so on. Incidentally, the miracle was in the mouth of the speaker, not the ears of the hearer.

Note also in verse 7 that the people were amazed. They recognized the apostles to be Galileans, apparently uneducated men, but able to speak fluently these many languages.

Now to your question. Peter begins speaking in verse 14 and addresses specifically the Jews who lived in that area. Furthermore, he told them to pay attention to his words. The language most commonly spoken in that place and time was Koine Greek, the language in which the New Testament was originally written. The record does not state, but this is the language he most likely would have used for that audience. It doesn't really matter what language he used, but we do know the Jews who killed Jesus a few weeks prior understood what he said (verse 37).

Nevertheless, as Peter was speaking, the foreigners present not knowing the language Peter used would not understand what he was saying. The record does not state this, but there is good chance that small groups could have formed again some distance away but within ear-shot of Peter. The other eleven could have been interpreting for them as Peter spoke. In this way, everyone at the gathering on that day could know what Peter was teaching the crowd. This would have at least been consistent with the purpose of the gift of interpretation (Corinthians 12:10, 14:5,26-28). It is also interesting to note that more teaching evidently continued throughout that day (Acts 2:40,41).