Speaking in the Congregation

It is a thing plainly repugnant to the Word of God, and the custom of the primitive Church, to have public prayer in the Church, or to administer the Sacraments in a tongue not understood by the people.

So also you, unless you uttered by the tongue words easy to understand, how would it be known what is spoken? For you would be speaking into the air.
— 1 Corinthians 14:9 (WEB)

Note

Originally directed at the Latin Mass. The principle is broader: worship should be intelligible to the worshipers.

Bible references

1 Corinthians 14:6-19 · 1 Corinthians 14:26-28 · Acts 2:6-11 · 1 Corinthians 14:9