en_tn/1co/14/16.md

you praise God ... you are giving thanks ... you are saying

Though "you" is singular here, Paul is addressing everyone who prays only in the spirit, but not with the mind. (See: rc://en/ta/man/translate/figs-you)

how will the outsider say "Amen" ... saying?

This can be a statement. AT: "the outsider will never be able to say 'Amen' ... saying." (See: rc://en/ta/man/translate/figs-rquestion)

the outsider

Possible meanings are 1) "another person" or 2) "people who are new to your group."

say "Amen"

"be able to agree" (See: rc://en/ta/man/translate/figs-synecdoche)