According to Acts 2:8, the believers spoke in languages that were understood by those visiting Jerusalem. The tongues are thus not angelic languages (see 1 Corinthians 13:1) but ordinary languages that were being used by people at the time.1 Where people would normally only know these languages if they were travellers who went to live in the different regions,2 the believers have been specially enabled in a miraculous way by the Spirit.
4 And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance.