Most of the NT references to the gift of foreign languages ("tongues") are found in 1 Corinthians 12, 13, & 14. The congregation at Corinth (in ancient Greece) was the most dysfunctional of all the NT congregations. Roughly 85% of the content of 1st Corinthians is corrective in nature; that is, it was written to correct (either directly or indirectly) specific, identifiable errors in belief and practice. The Corinthian believers were hindered by such things as divisiveness (1 Cor.1:10-17), carnality (1 Cor.3:1-3), immorality (including incest; 1 Cor.5:1-13), rampant divorce and remarriage (1 Cor.7:1-16), idolatry (1 Cor.8:1-10), social snobbery (1 Cor.11:17-34), and the abuse of spiritual gifts—especially tongues.

So how do we sort out Paul's teaching on tongues? Well, in 1 Corinthians 14, the KJV translators wisely made a distinction between an "unknown tongue" (singular, referring to gibberish) and "tongues" (plural, referring to coherent, foreign languages). They supplied the word "unknown" (in italics) in contexts where they understood that Paul was talking about unintelligible, nonsensical speech. They never connected "unknown" with tongues in the plural (that is, real foreign languages). I personally think it's unfortunate that this distinction has been lost in our modern translations (although, admittedly, it's an editorial and not a translational issue).

The singular "tongue" refers either (1) to ecstatic utterance or gibberish, or (2) to a singular, specific earthly language (for instance, today we would say "the English language," not "the English languages"), depending on the context. The plural "tongues," however, found in Chapters 12, 13, & 14, always refers in these chapters to genuine foreign languages. Paul encourages the gift of foreign languages; but he discourages any "unknown tongue," especially in public meetings. Here's how the distinction between "tongue" and "tongues" helps us decipher Paul's meaning in these chapters:

• 1 Cor. 12:10 — Here the word is plural. "Tongues" and "interpretation of tongues" are both listed as gifts from the Spirit given "for the profit of all" (v. 7). Remember that the ancient world was much more provincial than today's world. Most people never traveled more than a few miles from their birthplace and spoke only local dialects (of which there were hundreds across the Roman Empire). The gift of languages was an indispensable tool in carrying out the Great Commission.
• 1 Cor.12:28 — "Varieties of tongues," or foreign languages—again, plural. If tongues was the language of heaven, it wouldn't come in different varieties.
• 1 Cor.12:30 — "Do all speak with tongues?" Not everyone should expect to receive the gift of languages. Gifts are distributed among members of the Body (v. 12) according to each person's special calling and function (vv. 14-26). The gift of languages was mainly for evangelism and was therefore for the benefit of unbelievers. Prophecy, on the other hand, was for the benefit of believers (14:3-4).
• 1 Cor.13:1 — Paul talks here about "the tongues of men and of angels." This doesn’t necessarily make a distinction between the languages that men and angels speak. We use languages to communicate. The multiplicity of earthly languages is a consequence of God's judgment on humankind at the Tower of Babel (Gen. 11:9). Angels communicate with us in these languages. Whenever an angel speaks in Scripture, it's always in an earthly language (e.g., Gen. 21:17; Zech. 1:14; Rev. 8:13, 10:9).
• 1 Cor.13:8 — He says here that the miraculous gifts are (1) inferior to love and (2) impermanent. Prophecies (in the sense of divinely-imparted truths and/or foreknowledge) will "fail," tongues (plural; that is, divinely-enabled speech in foreign languages) will "cease," and knowledge (that is, according to Hodge, the “gift correctly to understand and properly exhibit the truths revealed by the apostles and prophets”) will "vanish away." Remember that when Paul wrote these words (around AD 55), the NT didn't yet exist in its final form. The only Bible they knew was the OT. Can you imagine being put in charge of a group of believers who've never seen a NT? You can see the problem—and that's why their leaders needed supernatural endowments and revelatory gifts of the Spirit to help them protect, teach, and nurture the flock. What Paul is saying here is that love is permanent, but the miraculous gifts (including tongues) will stop either when the NT is finalized ("that which is perfect has come," v. 10) or when the Lord returns (some commentators say the latter view better reflects the meaning of the Greek teleios, or "perfect").
• 1 Cor.14:2 — This is Paul's first use of the word "tongue" (singular) in this discussion about the use and misuse of spiritual gifts. He says that gibberish (or ecstatic utterance) is something only God could decipher, so it's better to prophesy in a language that people understand (see v. 1).
• 1 Cor.14:4 — Paul says if someone speaks in an unknown "tongue" (again, singular), he's not building up the other brothers and sisters in the congregation. Instead, he's only building up himself (and possibly attracting attention to himself), which Paul discourages.
• 1 Cor.14:5 — He says the gift of prophecy (i.e., spontaneous promptings to speak out boldly and declare God's truth) is greater than the gift of foreign languages and should have precedence in their public meetings. An exception, he says, is when an interpreter is present. In that setting, with a translator, the entire congregation can be edified through a message or teaching in a foreign language.
• 1 Cor.14:6 — Speaking in foreign languages (plural) is of limited value unless it's accompanied by corresponding teaching, prophecy, knowledge, or revelation (we'll define the latter term later) in whatever language the hearers understand.
• 1 Cor.14:9 — When we speak in a tongue (singular; that is, unintelligible utterance or gibberish), we are essentially "speaking into the air." That is, no one understands it and it accomplishes nothing.
• 1 Cor.14:13 — This is an exception to our rule that the singular "tongue" refers to unintelligible utterances. In this instance, Paul uses the singular word "tongue" because he's referring to someone who speaks in a singular foreign language (which would have made the plural, if he had used it here, grammatically incorrect). His point is that when we speak in a foreign language it should be interpreted so the hearers can benefit.
• 1 Cor.14:14 — "For if I pray in a tongue [singular], my spirit prays, but my understanding is unfruitful." The fact that Paul speaks in the first person here doesn’t mean that he spoke in gibberish any more than my saying, "If I rob a bank, I go to jail" means I committed the crime. This becomes clear in the next verse, where he says it's better to pray "with the spirit" and "with the understanding" (v. 15), than to have one without the other.
• 1 Cor.14:18 — Paul says, "I thank my God I speak with tongues more than you all." The Apostle traveled extensively, covering much of the first-century Roman Empire during his three (or possibly four) missionary journeys. He was well educated and no doubt knew several languages by training (at the minimum, Hebrew, Greek, and Aramaic) and may have exercised the gift of tongues to speak in other languages, as well.
• 1 Cor.14:19 — He says he would rather speak five understandable words than 10,000 words in an unknown tongue (singular). This is self-explanatory.
• 1 Cor.14:21 — Paul quotes here from Isaiah 28:11, where "another tongue" is the language of the Assyrians—an earthly language. Just as God spoke words of warning to unbelieving Israel through people who spoke a foreign language (Assyrian), the NT gift of foreign languages is also a sign to unbelievers. It’s intended to get their attention.
• 1 Cor.14:22 — Again, Paul contrasts the gift of foreign languages ("tongues," plural) with the gift of prophecy. Tongues is a sign to unbelievers (see the notes on 12:30 and 14:21 above), but prophecy is for believers, who are predisposed to receive God's truth.
• 1 Cor.14:23 — If an unbeliever comes into the congregational meeting and hears everyone speaking spontaneously and simultaneously in foreign languages, he might wonder if he’s stumbled into an insane asylum. But if everyone prophesies instead, the spontaneous proclamation of God’s message (although, again, that message is primarily directed to believers) may nudge that outsider toward a personal relationship with the Lord (vv. 24-25).
• 1 Cor.14:26 — There's a certain synergy that occurs when the various components of a public meeting complement each other. That is, someone sings a song, then another person offers a teaching from Scripture. If unbelievers are present, someone may share the Good News in the visitors' native language (provided that an interpreter is present so the whole group knows what's being said). In this verse, "tongue" is singular not because they're speaking gibberish, but because “each one” is speaking (that is, in one language at a time). Paul here says there might also be a "revelation" from someone as the Spirit of God begins peeling back layers of the proverbial onion and uncovering deeper applications of the truth of His Word. (The Greek word for "revelation" simply means to uncover something.)
• 1 Cor.14:39 — Paul says, "Do not forbid to speak with tongues." Evidently, some people in the Corinthian congregation were fed up with the confusion caused by people who were trying (without success) to speak in tongues (but only gibberish was coming out). They were ready to ban the practice altogether. Paul admonishes them, however, saying that they should allow the exercise of the legitimate gift of tongues. In other words, don't throw out the baby (the gift of speaking foreign languages) with the bathwater (a nonsensical tongue).