"For this reason anyone who speaks in a tongue should pray that he may interpret what he says. For if I pray in a tongue, my spirit prays, but my mind is unfruitful. So what shall I do? I will pray with my spirit, but I will also pray with my mind; I will sing with my spirit, but I will also sing with my mind. If you are praising God with your spirit, how can one who finds himself among those who do not understand say 'Amen" to your thanksgiving, since he does not know what you are saying? You may be giving thanks well enough, but the other man is not edified." NIV translation
Why do we, as Christians, pray aloud? In explaining "tongues", Paul helps us to see that we not only pray as an act of worship but also to build up others in Christ. He says that this idea of praying aloud in a "personal prayer language" where the mind is disengaged is not in keeping with the purpose of the spiritual gifts.
"I thank God that I speak in tongues more than all of you. But in the church I would rather speak five intelligible words to instruct others than ten thousand words in a tongue." NIV translation
Paul had traveled through many places that spoke foreign languages and, back in his day, they did not have all of the ways of learning a foreign language that we do. We never see that Paul had an interpreter but he was always able to share the gospel in a language that the locals could understand. This glorified God as everyone could see that it was clearly a gift from Him.
"Brothers, stop thinking like children. In regard to evil be infants, but in your thinking be adults. In the Law it is written: 'Through men of strange tongues and through lips of foreigners I will speak to this people, but even then they will not listen to me,' says the Lord." NIV translation
Paul quotes from Isaiah the words of the Lord that warned Jerusalem about their legalism and the fact that it would be foreigners (Gentiles) who would reveal the truth.
Once again, we see that this "speaking in tongues" was talking about the language of foreign countries.
"Tongues, then, are a sign, not for believers but for unbelievers; prophecy, however, is for believers, not for unbelievers. So if the whole church comes together and everyone speaks in tongues, and some who do not understand or some unbelievers come in, will they not say that you are out of your mind? But if an unbeliever or someone who does not understand comes in while everybody is prophesying, he will be convinced by all that he is a sinner and will be judged by all, and the secrets of his heart will be laid bare. So he will fall down and worship God, exclaiming, 'God is really among you!'" NIV translation
Paul gives a wonderful example of the purpose for the spiritual gifts and the result when they are properly used. In this, we see that God gives us the tools to use to bring people to faith in Him.