Following are a number of back-translations as well as a sample translation for translators of Isaiah 49:13:

• Kupsabiny: “Please, up there in heaven/sky sing for joy
  and down here on earth rejoice.
  Also the mountains should sing
  because God makes his people happy
  and he has mercy on those who are suffering.” (Source: Kupsabiny Back Translation)
• Newari: “Sing songs with great rejoicing, O heavens!
  Rejoice, O earth!
  Open your windpipe, sing songs exceedingly, O mountains!
  For the LORD has consoled his people,
  and he will have compassion on people who have fallen into sorrow and trouble.” (Source: Newari Back Translation)
• Hiligaynon: “You (sing.) shout for joy, O heavens! You (sing.) rejoice O earth! You (plur.) mountains sing! For the LORD will-have-mercy and make-happy his people who suffer.” (Source: Hiligaynon Back Translation)
• English: “Because of what Yahweh has promised to do,
  everything should shout joyfully—
  the sky and the earth and the mountains should sing,
  because Yahweh comforts his people,
  and he will pity those who are suffering.” (Source: Translation for Translators)