Here is great stuff from CORNELIUS À LAPIDE
Ver. 16.—Else when thou shalt bless with the spirit, &c. To bless here is to praise God with heart and mouth. S. Thomas understands it of the public blessing of the people; so also do Primasius, Haymo, and Salmeron, the latter of whom strives by many arguments to prove that the Apostle is speaking here of the sacrifice of the Mass, in which the priest blesses God rather than the people; for the two Greek words for “blessing” and “giving thanks,” used indifferently by the Evangelists and S. Paul in their accounts of the institution of the Eucharist, are used here, and seem to point to the Mass. It hence derives its names of the “Blessing” and the “Eucharist,” or giving of thanks. Add to this that in all the liturgies of the Mass, including those of S. James, S. Clement, S. Basil, and S. Chrysostom, after the consecration of the bread and wine, the people are wont to answer “Amen!” The Apostle, then, seems to mean here that public blessings, prayers, and Masses should not be celebrated in the church in an utterly unknown tongue, but that among the Greeks Greek should be used, among the Hebrews Hebrew, and among the Latins Latin; for these languages are for the most part understood by all who are of each race respectively. If it is impossible to use one language which is understood by all the different peoples who hear the same Mass, then one which is the best known should be selected, such as Latin among us, so that many “in the room of the unlearned” may answer “Amen!” as the Apostle requires.
The unlearned. Gagneius, following Severian, says the unlearned is the catechumen. Primasius says he is a neophyte. Chrysostom, Ephrem, Theophylact, S. Thomas, and others give the best meaning, viz., one untaught, unlettered, and with no knowledge of tongues.
S. Thomas, Primasius, and Haymo take the “unlearned” here to be the minister who at Divine service says “Amen!” for the people at the end of the Collects. These Fathers say that S. Paul means that at all events the minister at the Mass and other sacred rites should be able to understand the priest, or him who offers up prayer in public, in any other language than the vernacular, and should be able to respond, “Amen!” This is good and fitting teaching, but not necessarily the one uppermost in the mind of the Apostle.
But the “unlearned” here denotes, not some minister of the sacred rites, but any one of the laity. The Greek gives us, “he who sits among the unlearned” that is, is himself unlearned. Prophets and teachers used to sit in one place, the lay people in another. This is the explanation given by Chrysostom and Theophylact. Justin (Apol. 2) says that the whole of the laity, and consequently any individual of it, was wont to answer “Amen!” Hence S. Jerome, towards the end of his commentary on the Epistle to the Galatians, says that the people used to answer “Amen!” with a noise like thunder. A minister now says it for the people, so as to prevent a confused murmuring.
The Apostle is speaking here, we must notice once more, of the extempore prayer of the individual, uttered for the purpose of edifying, and which might possibly contain some doctrinal error, as is hinted in ver. 29. He directs that in such prayers the vulgar be used, so that the people may not answer “Amen!” to a prayer in an unknown tongue which is meaningless, absurd, or heretical. He is not speaking of prayers approved by the Church, which for that very reason are free from error, to which a single minister makes reply, and to which the people can add private prayers of their own. Moreover, the Council of Trent orders that sometimes, instead of the sermon, these prayers be explained to the people.
Again, it is lawful to pray in a language not understood by the person who prays, if you are certain that the prayers are good ones, as, e.g., when nuns say the Canonical Hours in Latin. In the same way the laity, when the priest offers up prayers in Latin, can pray with him, and add the intention of seeking that the priest may obtain for himself and all the people what he asks in the name of the Church in the beautiful prayers provided. And even if they do not understand them, and get no nourishment for their understanding from the meaning of the prayers, yet they reap the fruit of devotion to God, and of reverence towards the prayers; nay, they merit and obtain more than those who understand them if they pray with more humility, piety, and fervour.