The disciples did not actually speak in foreign languages. But God opened the ears of the Jews so that they heard the disciples speaking in their own languages (Acts 2:8,11). Ordinarily if several people speak in more than two languages at once, no one else can make out what is being said. On the day of Pentecost, however, 120 people all spoke in tongues. Jews from about 15 language groups were able to understand that they were â€œspeaking the wonderful works of Godâ€ in their own tongues (Acts 2:8-11).
The tongues were not intelligible to everyone in the crowd. While the devout Jews understood the tongues, others thought the disciples had too much wine (Acts 2:13). If the disciples were indeed speaking foreign languages, why then did only the devout Jews understand what was being said? And why did the ungodly think the disciples were drunk?
God intended to save the devout Jews and so allowed them to understand the tongues. Consequently, many believed and were baptized into Christ (Acts 2:37-41). The mockers, on the other hand, could not understand.