"I would like every one of you to speak in tongues, but I would rather have you prophesy. He who prophesies is greater than one who speaks in tongues, unless he interprets, so that the church may be edified." NIV translation
Paul has been speaking about unity in the body of Christ (see chapter 12) but now he seems to create division? What he is saying is that one who can teach the Word of God in a manner and language that people can understand is more valuable to the church than someone who speaks in a language that they cannot understand. This is unless someone can interpret (translate) the words for the people.
"Now, brothers, if I come to you and speak in tongues, what good will I be to you, unless I bring you some revelation or knowledge or prophecy or word of instruction. Even in the case of lifeless things that make sounds, such as the flute or harp, how will anyone know what tune is being played unless there is a distinction in the notes. Again, if the trumpet does not sound a clear call, who will get ready for battle? 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." NIV translation
Paul explains that the words have to have a meaning and not just some babble in an "unknown tongue". He compares the words to musical instruments and the fact that they have to have distinguishable notes to be useful. He also gives the example of sounding a trumpet for armies to go to war. He is reminding us that there is a purpose for all of the spiritual gifts and it is building up the body of believers. Many times this so-called "speaking in an unknown tongue" is used to beat down other people and make them feel unsure about their salvation. My friends, that is not of God but instead is a scheme of the devil.
"Undoubtedly there are all sorts of languages in the world, yet none of them is without meaning. If then I do not grasp the meaning of what someone is saying, I am a foreigner to the speaker, and he is a foreigner to me. So it is with you. Since you are eager to have spiritual gifts, try to excel in gifts that build up the church." NIV translation
Anyone who has been in a land where they do not speak the language can relate to what Paul is saying. All the languages of the world can be interpreted by someone or they would be useless. God has a purpose for everything that He does and the idea that He would have us babble in some unknown "prayer language" is a bunch of nonsense. There are those who hold up this notion of their own personal unknown prayer language but, as Paul has already said, the purpose of the spiritual gifts is to build up the church.