“And signs will follow to those believing these things: they will cast out demons in my name; they will speak new languages.”
If He was sending them to all areas of the known world to witness for Him, this should not be surprising. The answer is one of two options: either they would learn new languages, or He would give them the miracle of tongues as they needed, just as He did the disciples on that Pentecost. But in order to teach other people, it would have to be known languages, not something unutterable.
“And likewise the spirit also joins in to help our weakness; for we do not know what we should pray for as we ought, but the spirit himself pleads our case for us with groanings that cannot be uttered.”
“Groanings” are Strong’s #4726, the Greek stenagmos, meaning “a groaning, a sigh.”
Of course, these are unutterable. If you hear someone moan, groan, or sigh, how would you speak that in a word that other people would understand? You can’t! It’s simply a sound. But it conveys a feeling, an emotion. Yahweh can know that we feel depressed, sad, unhappy, worried, etc., simply by these sounds – and He knows what we need before we can even find the words to know how to ask.