Following are a number of back-translations as well as a sample translation for translators of Proverbs 15:21:

• Kupsabiny: “A foolish person is happy about foolishness,
  but an understanding person follows the way of uprightness.” (Source: Kupsabiny Back Translation)
• Newari: “A foolish person rejoices in his folly,
  but a wise person only does what is good.” (Source: Newari Back Translation)
• Hiligaynon: “A man without understanding rejoice in foolishness, but a man who has understanding is-living rightly.” (Source: Hiligaynon Back Translation)
• Kankanaey: “The mindless-person is-made-happy by foolishness that has no use/purpose, but the wise-person, what is does is what is right/correct.” (Source: Kankanaey Back Translation)
• English: “Foolish people are happy to continually act foolishly;
  those who have good sense do what is right.” (Source: Translation for Translators)