Exactly right re: 1 Cor 14:2. Although 'prayer language' is so cliche. If you are talking to God I guess that is prayer, and if you are talking in another language I guess that is a language, so you get "prayer language".
By the way , I don't buy into the pentecostal ideas of it being some secret language so that the devil can't understand you. If only God can understand what you say, then I guess what they say about the devil not understanding you is right. It's just that I dont think God gave the gift of tonges so you can pray without being understood by the devil, thats silly.
Let's consider what the mysteries are. Firstly, let's draw from the insight of some past noted baptist/presbyterian theologians:
Gill commentary says:
howbeit in the Spirit he speaketh mysteries; though under the influence and by the extraordinary gift of the Spirit he has, and to his own Spirit and understanding, and with great affection and devotion within himself, he speaks of the deep things of God, and the mysteries of his grace, the most glorious truths of the Gospel, yet the meaning of his voice and words not being known, he is a barbarian to them that hear him; and though what he delivers are truths of the greatest importance, they are a mere jargon to others, being unintelligible.
Barnes commentary says:
He speaketh mysteries - For the meaning of the word "mystery," see Note, 1Co_2:7. The word here seems to be synonymous with sublime and elevated truth; truth that was not before known, and that might be of the utmost importance.
So according to these men, by the use of the word 'mystery', it appears what Paul is saying is something more than merely not being able to understand what is being said. The content of the tongues must be something of value, elevated truth, the deep things of God - which I guess is why with interpretation it has great value to the edification of the church AND why tongues cannot be thought of as useless or irrelevant just because we have the bible.
It is a myth that the spiritual gifts were needed at the time 1 Corinthians was written because they did not have a bible. The fact is, they did have scripture, they circulated the writings of the apostles amongst the churches. They also had the old testament. For goodness's sake, don't you realise that 1 Corinthians itself is the bible and they obviously had access to it??Are we so stupid to think just because they didnt have a completely bound KJV that the early church were somehow lacking?
They had Scripture, they had personal accounts and witnesses of Jesus's resurrection. They had the presence of the apostles themselves!!! Yet they also needed the spiritual gifts and did not reject them!! Yet today, even though we have less than the early church had, we reject the gifts just because we have a bible , and that is completely pointless and stupid.
"Who needs the gifts when you can just read the bible?", someone will say.
Well there's a few reasons why. Firstly, just reading the bible won't give you the deep things of God, as the apostle Paul had, exceedingly great revelations. He didn't get that by reading the old testament and consulting his favorite theologian. He got that by the Spirit speaking to and through him.
Secondly, when you just read the bible, you decide what you read. Perhaps you want to read John but God wants to speak a word from Psalms in and through you?
Thirdly, use of the gifts shows reliance upon the Holy Spirit. God did not intend us to be just reading the bible without any experience or fellowship with the Spirit.