Following are a number of back-translations as well as a sample translation for translators of Joel 3:21:

• Kupsabiny: “I will repay/avenge the blood of the people of Judah
  and not forgive the sinners.
  I God shall live in Zion.’” (Source: Kupsabiny Back Translation)
• Newari: “Their guilt of having shed blood that I have not yet forgiven I will forgive."
  The LORD is dwelling in Zion.” (Source: Newari Back Translation)
• Hiligaynon: “I will-take-revenge-upon the ones who have-killed my people; I will- surely -punish the one who has-done this to them. I (am), the LORD who lives in Zion.’” (Source: Hiligaynon Back Translation)
• Kankanaey: “I will avenge those whom they killed, I will not forgive those who sinned. But in Juda and Jerusalem however, there-will-be in-the-future inhabitants forever, and I who am God, I will stay on mountain Zion.’” (Source: Kankanaey Back Translation)
• English: “I, Yahweh, live on Zion Hill in Jerusalem,
  and I will get revenge on the people of Egypt and Edom who killed many of my people.’” (Source: Translation for Translators)