Speaking in tongues is mentioned in several passages in the New Testament: Mark 16:17; Acts 2:1-13; 10:44-46; 19:6; 1 Cor 12-14. It is important to understand that the Greek word for “tongue” was the same word used for “language”, so “speaking in tongues” means “speaking in languages”.
Most of the incidents in the New Testament are clearly real languages:
Mark and Acts – real languages “in our own tongues”
The reference in Mark 16:17 is to speaking in “new tongues”, which may mean languages previously unknown to the apostles.
In Acts 2 the reference is clearly to human languages, with many different peoples hearing the apostles in their “own tongues” (v11).
(both Jews and converts to Judaism); Cretans and Arabs—we hear them declaring the wonders of God in our own tongues!”
Acts 10:44-46 is uncertain; speaking in tongues is a sign of the pouring of the Holy Spirit but we are not told whether they spoke human languages or something else. The same is true of Acts 19:6.
But there is one exception:
Corinth – tongues that “no one understands”
In 1 Corinthians 12-14 we have the fullest account of speaking in tongues. When writing about speaking in tongues Paul compares it with speaking a foreign language: “if I do not know the meaning of the language, I shall be a foreigner to him who speaks” (14:11). He also commands that if they are to speak in tongues in the church then there should be an interpreter present (14:27-28), which implies that languages were being spoken that could be interpreted.
This being said, there are some other aspects to speaking in tongues. Paul refers to speaking with the tongues “of angels” (13:1) and says that one who speaks with a tongue “does not speak to men but to God” (14:2). However we should be cautious about reading too much into these statements.
One explanation is that Corinth had those who were practising or experiencing glossolalia. Another is that these were ‘tongues of angels’ as found in the Dead Sea Scrolls and in Alexandrian Jewish texts. A kind of adlibbed music, or possibly songs in Hebrew. Another is there was simply real language confusion in Corinth. Or the sum of all three of these suggestions. Either way, Paul forbids the use of tongues of any kind without translation.