I Corinthians 14:15 "What is it then? I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also."
He continues..
Corinthians 14:16 "Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say "Amen" at thy giving of thanks, seeing he understandeth not what thou sayeth?"
I Corinthians 14:17 "For thou verily givest thanks well, but the other is not edified."
This is all about going into a church and speaking and praying in a different language that those do not understand. It's not edifying (building up) the church.
I Corinthians 14:18 "I thank my God, I speak with tongues (languages in the Greek) more than ye all:"
Paul could speak more than one language
I Corinthians 14:19 "Yet in the church I had rather speak five words with my understanding, that by my voice I might teach others also, than ten thousand words in an unknown tongue."
And then we come full circle to his main point.