There is more than just one type of tongue.
Paul wrote that there are DIVERSITIES of tongues, meaning more than just one kind:.
1Co 12:10 to another the working of miracles, to another prophecy, to another the ability to distinguish between spirits,
to another various kinds of tongues,
to another the interpretation of tongues.
So what you had boldened is about the variety of foreign languages because what follows is the interpretation of those tongues.
There’s a tongue found in Acts 1 that’s understood by everyone in their own language, needing no interpreting.
That is the real God's gift of tongues, however when used in the assembly, it will come with interpretation because the manifest of the gifts are to profit the body of Christ.
There’s an unknown tongue that’s never understood by any man, because it’s the Holy Spirit speaking mysteries unto God.
Are there mysteries to God? No. Read that again.
1Co 14:2 For he that speaketh in an unknowntongue speaketh not unto men, but unto God: for no man understandeth him;howbeit in the spirit he speaketh mysteries.
It is man that does not understand what he is saying, so what he is speaking is a mystery to those around him. But read that in context;
1Follow after charity, and desire spiritual gifts, but rather that ye may prophesy. 2 For he that speaketh in an unknown tongue speaketh not unto men, but unto God: for no man understandeth him; howbeit in the spirit he speaketh mysteries.
That is Paul explaining why prophesy is better than all the gifts, especially tongues because tongues is not a stand alone gift to use in the assembly.
There’s an unknown tongue spoken to the congregation in church, that is a message from God that needs interpreted by someone, that Paul calls prophecy.
Prophesy which is edification, is a separate function in the church from tow or 3 speak in tongues which another is to be interpreted. When it comes to prophesying, it is the same as two or three prophesy and one judges it.
1Co 14:27 If any man speak in an unknowntongue, let it beby two, or at the most bythree, and thatby course; and let one interpret.
28 But if there be no interpreter, let him keep silence in the church; and let him speak to himself, and to God. 29 Let the prophets speak two or three, and let the other judge. 30 If any thing be revealed to another that sitteth by, let the first hold his peace. 31 For ye may all prophesy one by one, that all may learn, and all may be comforted.
One can get the order & decency in the church because when two or three speak in tongues or prophesy, they do so one by one.
Why did Paul say if there is no interpreter, to make that person be silent in church? This is Paul explaining that is a foreign visitor speaking out of turn during this function where 2 or 3 speak in tongues for that other to interpret. That is why there is no interpretation to that person because he is not speaking in God's gift of tongues. He is speaking in the language he understands as that is what Paul mean that he is speaking to himself and to God as God understands him too, not that the man is a nut speaking to himself and to God in verse 28
That is why in verse 2 when he speaks unto God, God understands what he is saying because that tongue is manifested by the Holy Spirit; not that the tongue speaker is speaking to God but unto men whereby a the moment he speaks in mysteries for why prophesy is better than tongues among all spiritual gifts because tongues is not a stand alone gift to use in the assembly for why it will come with interpretation.
There’s an unknown tongue that isn’t interpreted, that is the Holy Spirit praying for the person speaking in tongues, for things they know not how to pray for.
Rom 8:26 Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us
with groanings which cannot be uttered.
That means no sound at all. This is a testimony about the Holy Spirit for
all believers that do not know what to pray for so that is why it is not involving a supernatural tongue at all. Jesus is he he in verse 27 that searches our hearts & knows the mind of the Spirit for how the silent intercessions of the Spirit is known to the Father because Jesus gives the silent intercessions of the Spirit to the Father, being our only Mediator between God and men which is in according to the will of God for Him to do ( 1 Timothy 2:5 & Hebrews 7:25 )
Matthew 6:7 But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking. 8 Be not ye therefore like unto them:
for your Father knoweth what things ye have need of, before ye ask him.
BTW, Paul ordered the Corinthian church not to forbid speaking in tongues: If the only kind of tongue was the type found in Acts 1 (that is understood by everyone), this means that church had forbidden speaking in known languages - which would have resulted in very quiet church services!
Paul instructed that if there is no interpreter, he is to be silent because he is not really speaking in God's gift of tongues but in his own language.
As for any notion for private use, believers should have rea 1 Corinthians 12th chapter that proves none of the gifts are for private use.
1 Corinthians 12:4
Now there are diversities of gifts, but the same Spirit. 5 And there are differences of administrations, but the same Lord. 6 And there are diversities of operations, but it is the same God which worketh all in all.
7 But the manifestation of the Spirit is given to every man to profit withal.
11 But all these worketh that one and the selfsame Spirit, dividing to every man severally as he will. 12 For as the body is one, and hath many members, and all the members of that one body, being many, are one body: so also is Christ. 13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit. 14 For the body is not one member, but many.
The scripture above testifies that there is only one baptism with the Holy spirit as that had occurred at our salvation so there is no other baptism with the Holy Ghost with evidence of tongues because that is an apostate calling for why that tongue is gibberish nonsense that cannot come with interpretation. So believers wanted to believe it is of God without testing the spirits and so they just go with the flow of that supernatural tongue, not knowing what benefit that is being manifested by that tongue; hence unwittingly making God he author of confusion, but He is not, therefore that tongue for private use gained by that extra phenomenon is why it is not of Him at all.
Below is proof that the real God's gift of tongues would never be for private use because that would be the same as needing no interpreter or any other body of Christ.
19 And if they were all one member, where were the body? 20 But now are they many members, yet but one body. 21 And the eye cannot say unto the hand, I have no need of thee: nor again the head to the feet, I have no need of you.
Better to shun that tongue for private use, pray normally so you know what you had prayed for to give the Father genuine thanks in Jesus's name.