When the apostles were speaking in tongues, were they actually speaking a different language or did each person hear them in their own language? Were they each speaking and different groups congregated toward the one who was speaking their particular language or did each apostle just speak and every person just heard him in their own language?
When the people exclaimed about the miracle, they said "we hear them speaking in our languages the mighty works of God!" (Acts 2:11). Because it says "them" we know that more than one apostle was speaking. This fits with what is stated next, "But Peter, standing up with the eleven, lifted up his voice, and spoke out to them" (Acts 2:14). It was Peter's sermon, but the other eleven were also there, presumably translating what Peter said into other languages. The people heard the apostles speaking in their native languages. This is who the Spirit described the miracle. "They were all filled with the Holy Spirit, and began to speak with other languages, as the Spirit gave them the ability to speak" (Acts 2:4). The miracle was that they spoke in other languages, not that people heard in other languages.