In the ancient church till today there was a gift of the Spirit of prayer. Today its refered to as prayer of the heart sometimes as contemplative prayer. Its unceasing prayer and it is what Paul is speaking when he refers to praying with his spirit (v 15).
John Chrysostom in 390 AD a famous preacher from Antioch and native speaker of koine greek explains this practise as it occured a few generations earlier and was still remembered by the Church:
For if I pray in a tongue, says he, my spirit prays, but my understanding is unfruitful. Do you see how by degrees bringing his argument to a point, he signifies that not to others only is such a thing useless, but also to himself; if at least his understanding is unfruitful?
For if a man should speak only in the Persian, or any other foreign tongue, and not understand what he says, then of course to himself also will he be thenceforth a barbarian, not to another only, from not knowing the meaning of the sound. For there were of old many who had also a gift of prayer, together with a tongue; and they prayed, and the tongue spoke, praying either in the Persian or Latin language , but their understanding knew not what was spoken. Wherefore also he said, If I pray in a tongue, my spirit prays, i.e., the gift which is given me and which moves my tongue, but my understanding is unfruitful."(homily 35 on 1 Corinthians)
As you can see these people spoke authentic languages. John Chrysostom goes on to explain 1Cor 14.16 and explains what is meant by the room of the 'unlearned'. Furthermore John Chrysostom explains that Paul used a literary technique where he downplays the gift of tongues but at the same time elevates it (he demostrates this throughout his commentary on chapter 14):
...Now by the unlearned he means the layman, and signifies that he also suffers no little loss when he is unable to say the Amen. And what he says is this: if you shall bless in a barbarian tongue, not knowing what you say, nor able to interpret, the layman cannot respond the Amen. For not hearing the words, 'forever and ever,' which are at the end , he does not say the Amen. Then again, comforting them concerning this, that they might not seem to hold the gift too cheap; the same kind of remark as he made above, that he speaks "mysteries", and "speaks unto God", and edifies himself, and prays with the spirit, intending no little comfort from these things, this also he utters here, saying, for thou indeed givest thanks well, since you speak being moved by the Spirit: but the other hearing nothing nor knowing what is said, stands there, receiving no great advantage by it.
Most Laymen were common people 'unlearned' so they could not understand the authentic language without an interpreter, they were untrained in multiple languages. If this was ecstatic utterance it would be irrelelvant whether the room was full of 'unlearned' people as whatever was being spoken would never have been understood even by one who knew 10 languages. Likewise it would be irrelevant to know when to say AMEN as ecstatic utterance would not be offering a doxology.
