The word "tongue" simply means language. The Greek word is Strong's #1100, "glossa" and is defined as the tongue, a language. (1) It does not mean some mystical utterances.
An unknown tongue means a foreign language not known by the congregation. If there is no one to interpret that foreign language then it only edifies the one speaking.
"for he who is speaking in an [unknown] tongue -- to men he doth not speak, but to God, for no one doth hearken, and in spirit he doth speak secrets;" (1 Cor. 14:2, YLT)
The word in brackets [unknown] is not in the original, but was added. The original text just says "speaking in a tongue".
The Holman Christian Standard Bible translates it as:
"For the person who speaks in another language is not speaking to men but to God, since no one understands him; however, he speaks mysteries in the Spirit."
Contemporary English Version:
"If you speak languages that others don't know, God will understand what you are saying, though no one else will know what you mean. You will be talking about mysteries that only the Spirit understands."
International Standard Version:
"For the person who speaks in a foreign language is not actually speaking to people but to God. I..."
Speaking in tongues does not mean some unintelligible sounds spilling forth from the mouth. Acts chap. 2 makes it clear that speaking in "tongues" was speaking a foreign language.
"3 and there appeared to them divided tongues, as it were of fire; it sat also upon each one of them, 4 and they were all filled with the Holy Spirit, and began to speak with other tongues, according as the Spirit was giving them to declare....
"6 and the rumour of this having come, the multitude came together, and was confounded, because they were each one hearing them speaking in his proper dialect, 7 and they were all amazed, and did wonder, saying one unto another, `Lo, are not all these who are speaking Galileans? 8 and how do we hear, each in our proper dialect, in which we were born? (Acts 2:3-4, 6-8, YLT)
The Holy Spirit poured forth the ability to speak in other dialects / languages so that the unbelieving could believe, just as were the other powers of the spiritual gifts.
Paul explained further in 1 Cor. 14:22-23 that:
"22 so that the tongues are for a sign, not to the believing, but to the unbelieving; and the prophesy [is] not for the unbelieving, but for the believing,
23 If, therefore, the whole assembly may come together, to the same place, and all may speak with tongues, and there may come in unlearned or unbelievers, will they not say that ye are mad?" (YLT)
If everyone of the assembly spoke in an unknown / foreign language and some unbeliever came in among them, then the unbeliever would not be able to understand them, and would think they were crazy. Which is why Paul goes on to say in vs. 27,
"27 if an [unknown] tongue any one do speak, by two, or at the most, by three, and in turn, and let one interpret;" (YLT)
The word "interpret" is Strong's Gr. 1329, and means to explain, or translate. It means to translate from one language to another language. (2)
In vs. 21, Paul references Isaiah.
"in the law it hath been written, that, `With other tongues and with other lips I will speak to this people, and not even so will they hear Me, saith the Lord;'" (1 Cor. 14:21, YLT) ... which is from...
"Very well then, with foreign lips and strange tongues God will speak to this people,..." (Isa. 28:11, NIV)
"For by people of strange lips and with a foreign tongue the Lord will speak to this people," (ESV)
"So with stammering lips, in a foreign accent, [Adonai] will speak to this people." (CJB)
Isa. 28:11 prophesied that God would speak to them in foreign languages. As Paul referenced Isa. 28 in 1 Cor. 14 then the context of 1 Cor. 14 must be about foreign languages known to men. See also the discussion of this issue at BlueLetterBible by Don Stewart.
Therefore, if no one else in the assembly speaks that foreign language, then the speaker in "tongues" is only edifying himself.
Strong's Gr. 1100, glossa - Biblehub
Strong's Gr. 1329, dierméneuó - Biblehub