Following are a number of back-translations as well as a sample translation for translators of Isaiah 59:21:

• Kupsabiny: “God is saying, ‘I am making my covenant with you. You (plur.) have my spirit and I have given you my words. From today and forever, those words shall never leave your mouths. You shall take those words to heart and speak about them from today and forever. They shall never leave your mouths those of your grandchildren and those of your descendants/great-grandchildren.’” (Source: Kupsabiny Back Translation)
• Newari: “The LORD says, "As for me, this will be my covenant for you. You have my Spirit, and it will not depart from you. I have put my words in your mouth. and it will be in your mouths and in the mouths of your children and in the mouths of your descendants always from now on.
  The LORD has declared [it].” (Source: Newari Back Translation)
• Hiligaynon: “And this is my covenant with you (plur.): ‘Never ever my Spirit which is in you (plur.) will-disappear and my words which I caused- you (plur.) -to-speak. You (plur.) declare/tell this to your (plur.) children, and then they will- also -tell this to their children up to the future generation. I, the LORD, (am) the-(one) saying this.’” (Source: Hiligaynon Back Translation)
• English: “This is what Yahweh says to his people: ‘This is the agreement that I will make with you:
  My Spirit will not leave you,
  and you will always have my message.
  You will be able to declare it ,
  and your children and grandchildren will be able to declare it forever.’” (Source: Translation for Translators)