1 Cor 14:
1 Follow the way of love and eagerly desire gifts of the Spirit, especially prophecy.
2For anyone who speaks in a tonguea does not speak to people but to God. Indeed, no one understands them; they utter mysteries by the Spirit. 3But the one who prophesies speaks to people for their strengthening, encouraging and comfort.
4Anyone who speaks in a tongue edifies themselves, but the one who prophesies edifies the church. 5I would like every one of you to speak in tongues,b but I would rather have you prophesy. The one who prophesies is greater than the one who speaks in tongues,
c unless someone interprets, so that the church may be edified.
People who claim modern tongues are real mostly base their arguments on the two verses (1 Cor 14:2/4,5). Despite these verses, tongues should be real human language, not unknown modern tongues. Paul did not mean that no one understands tongues. I'll explain with an example:
Language is a tool used to transfer understanding from one person to the other and when we preach the gospel, we are trying to transfer what we know to others who don't know and make them believe.
I can be given the ability to utter Greek, but if i utter Greek to a Spanish congregation, then i'm speaking mysteries to them and
my understanding is unfruitful because i'll not manage to transfer my understanding to others who truly need it. For these reason, Paul says you are speaking mysteries in spirit because no one understands and that your understanding doesn't bear fruit- better if you keep quiet.
It is true that if you are not successful in transferring understanding to others (edifying others), the edification (understanding) remains with you and that's what Paul calls self edification. The gifts of the Holy spirit are never for self edification but should profit all within the body.
And Paul was correcting the Corinthians for misusing the gift of tongues; yes it could have been real but the idea that they'd go speaking Spanish to Greeks was childish.
And Paul told them:
1 Cor 14:
9 So it is with you.
Unless you speak intelligible words with your tongue, how will anyone know what you are saying? You will just be speaking into the air.
So in essence, we must speak Spanish to a Spanish congregation.
Modern tongues are not even a language, they go against 1 Cor 14:9 where we are told, we must speak intelligible words with our tongue.