Help me out here
@JonC. Are you saying that you believe that an English speaking person goes alone into their prayer closet and suddenly speaks German in prayer because the Holy Spirit is making them speak German just so that they can’t understand it?
No. That would be expressions that can be expressed in words (you are just shifting languages).
There are two topics here. One is speaking in tongues with another interpreting as a sign for the unbeliever. We do not know that this was a known language. It could have been. But that is not what I am talking about.
I am talking about Paul's words (God's words) regarding private prayer.
God tells us that when a Christian prays with utterances that express what cannot be expressed in words, this "intercession of the Spirit, he is to do so in private and his spirit is edified but his mind is not.
You keep asking for an explanation. Paul gives none. This cannot be expressed in words.
God says it. We can believe it or reject it.
You choose to reject it because you lack understanding (it is experiential by definition and you lack the experience).
Others also do not understand it, yet they believe it because God says it is so.
I draw a comparison but also do not fully understand this topic. I believe I have been on the cusp a few times in prayer (I caught myself mumbling while praying for what I cannot express).
But I think all Christians have. We do not always know what to ask or the path forward. I do not know of a Christian who has not had that type of experience, so we do have something to compare it to.
But in this case, God tells us nobody understands the utterances because it is too "deep" to be expressed in words. Nobody is there to hear the sounds made (this is why it is called "private").
Ultimately you have 1 Corinthians 14. You seem to like parts but reject about half of God's words on this topic. We do not have to pray in "tongues", but we cannot call God a liar because we lack that experience.