Albert Barnes on 1 Corinthians 14:19

Yet in the church. In the Christian assembly. The word church does not refer to the edifice where Christians worshipped, but to the organized body of Christians.

I had rather, etc. It is probable that in the Christian assembly, usually, there were few who understood foreign languages. Paul, therefore, would not speak in a foreign language when its only use would be mere display.

With my understanding. So as to be intelligible to others; so that I might understand it, and so that at the same time others might be benefited.

(+) "understanding" "So as to be understood" (++) "unknown tongue" "Language"