Here the word "tongue" refers to the person who speaks. Alternate translation: "Wise people compliment knowledge when they speak" (See: [[rc://en/ta/man/translate/figs-synecdoche]])
The writer speaks of fools' mouths as if they were containers and of folly as if it were the liquid that filled them. When fools speak, their mouths pour out the liquid. The word "mouth" represents those who speak. Alternate translation: "fools are always speaking folly" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-synecdoche]])