For thou verily givest thanks well, but the other is not edified.

Jamieson-Fausset-Brown Bible Commentary

17. givest thanks-The prayers of the synagogue were called "eulogies," because to each prayer was joined a thanksgiving. Hence the prayers of the Christian Church also were called blessings and giving of thanks. This illustrates Col 4:2; 1Th 5:17, 18. So the Kaddisch and Keduscha, the synagogue formulæ of "hallowing" the divine "name" and of prayer for the "coming of God's kingdom," answer to the Church's Lord's Prayer, repeated often and made the foundation on which the other prayers are built [Tertullian, Prayer].

Matthew Henry's Concise Commentary

14:15-25 There can be no assent to prayers that are not understood. A truly Christian minister will seek much more to do spiritual good to men's souls, than to get the greatest applause to himself. This is proving himself the servant of Christ. Children are apt to be struck with novelty; but do not act like them. Christians should be like children, void of guile and malice; yet they should not be unskilful as to the word of righteousness, but only as to the arts of mischief. It is a proof that a people are forsaken of God, when he gives them up to the rule of those who teach them to worship in another language. They can never be benefitted by such teaching. Yet thus the preachers did who delivered their instructions in an unknown tongue. Would it not make Christianity ridiculous to a heathen, to hear the ministers pray or preach in a language which neither he nor the assembly understood? But if those who minister, plainly interpret Scripture, or preach the great truths and rules of the gospel, a heathen or unlearned person might become a convert to Christianity. His conscience might be touched, the secrets of his heart might be revealed to him, and so he might be brought to confess his guilt, and to own that God was present in the assembly. Scripture truth, plainly and duly taught, has a wonderful power to awaken the conscience and touch the heart.