The tongues described in the Bible are known and used languages. Every tongue is a language that has meaning to ordinary people somewhere in the world.
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. (1 Corinthians 14:10-11)
The passage in chapter two of Acts pictures Jews who came from every country under heaven and gathered to listen to the apostles tell about the gospel of Jesus Christ. When the apostles spoke, each man clearly heard and understood the gospel message in his native spoken language –not mere babbling, but in his own familiar language.
Now there were staying in Jerusalem God-fearing Jews from every nation under heaven. When they heard this sound, a crowd came together in bewilderment, because each one heard them speaking in his own language. Utterly amazed, they asked: “Are not all these men who are speaking Galileans? Then how is it that each of us hears them in his own native language? Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome (both Jews and converts to Judaism); Cretans and Arabs– we hear them declaring the wonders of God in our own tongues!” (Acts 2:5-11)
There is a passage in Isaiah 28 describing the lengths that God would go to in order to bring the rebellious nation of Israel to salvation.
Very well then, with foreign lips and strange tongues God will speak to this people, to whom he said, “This is the resting place, let the weary rest”; and, “This is the place of repose” —but they would not listen. So then, the word of the LORD to them will become: Do and do, do and do, rule on rule, rule on rule; a little here, a little there —so that they will go and fall backward, be injured and snared and captured. (Isaiah 28:11-13)
God explained that He will reach His people using foreigners and the heavy burden of the law to do it. Now here in this 1 Corinthians 14 passage, Isaiah 28 is quoted to plainly lay out the purpose of tongues. Speaking in tongues is a sign to the unbelieving Jew and it started at Pentecost. God’s purpose was to communicate the Gospel. It’s to bring the lost Jews to Christ.
In the Law it is written: “Through men of strange tongues and through the lips of foreigners I will speak to this people, but even then they will not listen to me,” says the Lord. Tongues, then, are a sign, not for believers but for unbelievers; prophecy, however, is for believers, not for unbelievers. (1 Corinthians 14:21-22)
Tongues are not for the saved, especially not in their assemblies. In fact, if an unbeliever were to walk in while someone was speaking in a tongue (a foreign language), he would think the whole group to be crazy.
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? (1 Corinthians 14:23)
There is another way for believers to communicate in their assembly: Prophecy. That’s merely speaking what God has told us through His Son. –It’s all recorded in the Bible.
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!” (1 Corinthians 14:23-25)