Christ’s Apostle Paul made clear that tongues are unintelligible to men without the miraculous Gift of Interpretation of Tongues:
For he who speaks in a tongue does not speak to men but to God, for no one understands him (1 Corinthians 14:2)
Luke, not an Apostle but a historian, merely described what happened in Acts 2 but did not explain it.
From the Apostle Paul’s instruction in 1Corinthians 14, we understand that in Acts 2 we see both TONGUES and INTERPRETATION OF TONGUES on display: The 120 spoke publicly in languages that seemed like the speech of drunks to the local Judeans who mocked it. But the same speech was received as intelligent communication to others “from every nation under heaven” who were experiencing the GIFT OF INTERPRETATION OF TONGUES.
In short, Tongues are not understandable by anyone without the equally miraculous Gift of Interpretation of Tongues.
“to another different kinds of tongues, to another the interpretation of tongues. 11 But one and the same Spirit works all these things”
(1 Corinthians 12:10–11, NKJV)
“And the spirits of the prophets are subject to the prophets.”
(1 Corinthians 14:32, NKJV)
This is true of anyone who speaks in tongues, as well, as the context surrounding this verse makes clear.
So, it is contrary to the Apostle Paul’s teaching that speaking in tongues must be involuntary.
“If anyone speaks in a tongue, let there be two or at the most three, each in turn, and let one interpret. 28 But if there is no interpreter, let him keep silent in church, and let him speak to himself and to God.”
(1 Corinthians 14:27–28, NKJV)
To “speak to oneself and to God” is called prayer.
So, Christ’s Apostle Paul clearly taught here that there is such a thing as praying in tongues.
Same is true of saying a prayer of thanksgiving in tongues (“bless with the spirit”),
“Otherwise, if you bless with the spirit, how will he who occupies the place of the uninformed say “Amen” at your giving of thanks, since he does not understand what you say? 17 For you indeed give thanks well, but the other is not edified.”
(1 Corinthians 14:16–17, NKJV)