Bible Verses About Public Prayer: Should not be made in an unknown language

For if I pray in another language, my spirit prays, but my understanding is unfruitful.

What is it then? I will pray with the spirit, and I will pray with the understanding also. I will sing with the spirit, and I will sing with the understanding also.

Otherwise if you bless with the spirit, how will he who fills the place of the unlearned say the “Amen” at your giving of thanks, seeing he doesn’t know what you say?