Some have tried to categorically state that those who came out from the upper room that day supernaturally spoke in known human languages. But Paul specifically wrote that speaking in tongues is not speaking in a known language. In First Corinthians 14:2, Paul said, “For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him.…” That word unknown is italicized in the King James Version because it doesn’t appear in the Greek text; it was supplied by the translators to affirm that Paul was not referring to known human languages; he was writing about a supernatural prayer language that is known only to God.
According to Paul’s words in First Corinthians 14:13-15, speaking in tongues is a spiritual language — never a known language. It is so supernatural and unknown to man that it cannot be understood, not even by the speaker himself, unless he prays for the ability to interpret what he is saying. Since this is Paul’s very clear teaching about speaking in tongues, it emphatically asserts that on the Day of Pentecost, the believers did not speak in known human languages, but in a supernatural, unknown prayer language, just as believers speak in tongues today.
However, on the Day of Pentecost, a special miracle occurred. The believers spoke in tongues — but by the time that supernatural language reached the ears of the listeners, they “heard” a message in their own distinct dialects. This is why Acts 2:6 says that “…every man heard them speak in his own language.” Acts 2:8 says, “And how hear we every man in our own tongue, wherein we were born?” And Acts 2:11 tells us that the listeners said, “…we do hear them speak in our tongues the wonderful works of God.”
On the Day of Pentecost, the first great work of God was to fill the believers with the Holy Spirit. The second great work was to liberate their human spirits so they could pray in tongues and worship God in the Spirit. The third great work occurred when the believers spoke in other tongues and God supernaturally translated those tongues in the ears of the listeners so that each member of the audience “heard” them speaking in his or her own language