“For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries” (1 Cor. 14:2).
The gift of tongues is an exercise in mystery. A man speaking in tongues is a man who is speaking mysteries in his spirit. It is a mystery because the language is unknown to those present, and unknown to him.
Because he is speaking to God, we know that God understands him. This means that the language is unknown, not that it is unknowable. It is an unknown tongue, which is not the same thing as gibberish.
When the disciples spoke in tongues at Pentecost, it happened that many foreign speakers were present in Jerusalem because of the festival. What Paul says here about tongue-speaking Corinth was not the case in Jerusalem. They began speaking in other tongues (glossa, Acts 2:4), and when a crowd gathered, they heard them speaking in their own languages (dialektos, Acts 2:6). We get the word dialect from that word. They were speaking in known languages.
The saints in Corinth were doing the same thing, but the languages were not known to anyone on the premises—we will learn what the point of that was a bit later in the chapter.