"no man understands" refers to the one speaking in tongues - It is NOT a known language to the one speaking. Not to those in the church meeting. The "him" in the KJV is added. That is why it was so remarkable that the apostles spoke in the languages of the people present when they spoke in tongues . . . that is why the multitude were "confounded" when they heard them speak.
Not sure how one could ever make that conclusion – it does not refer to the one speaking but rather to those listening; there is no other way that can be interpreted/translated. Yes, the ‘him’ is added – it’s more or less understood/inferred from the context of the whole sentence.
People (at Pentecost) were confounded because they were expecting to hear Hebrew; the socially/religiously correct language to use on that occasion. What they heard and what confounded them was that they were hearing their own languages (only two were spoken by all Jews present) – Greek and Aramaic.
NASB - For one who speaks in a tongue does not speak unto men, but to God; for no one understands, but in his spirit he speaks mysteries (secrets).
The original Greek has: pneumati de, lalei mystêria – “in spirit however, he utters mysteries”. No “his”. Though he’s praying earnestly and from the heart (i.e. in the spirit) he’s speaking ‘mysteries’ to those listening to him as they have no clue what he’s saying; to them his speech/language is a mystery.
Tongues are NOT meant to be used to "prove a point" to someone.
Though that may be true, tongues can in fact be studied and analyzed. That they are languages…or not, is a very testable claim.
Speaking in tongues is a language.
Language, whether spoken somewhere on this planet, an alien world, or in the spirit realm (heaven, by angles, etc.) are going to consist of certain elements which all language must have to be considered ‘language’; tongues simply does not meet any of these criteria.
And forbid not to speak with tongues (read: languages)
I think this makes perfect sense given that Paul was, after all, in the process of founding a religion (either wittingly or not). In order for it to spread, the more people that had access to its message, the better its success.
What better way to spread the word than to do it in people’s native languages rather than in some prescribed language (e.g. Judaism was originally restricted to Hebrew, Zoroastrianism is still to this day restricted to Avestan, etc.). Not restricting a religion by allowing access to it in only one language was a very novel idea/concept at this time.
In this sense, Paul is spot on; don’t forbid speaking other languages (it’s the best way to spread this new religion called Christianity). A great way to people to discuss it and practice it. In order to spread the word though, you may need to “speak with new tongues”, i.e. you’ll need to learn languages you probably didn’t even know existed in order to spread the message to people.