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

• Kupsabiny: “The people of Israel are crying
  and they are pleading on the mountain
  because sin has made them get lost
  and they have forgotten me their God.” (Source: Kupsabiny Back Translation)
• Hiligaynon: “‘Noise is-heard on the top of the clear mountains. The people of Israel are-weeping and pleading. For their ways are wicked and they forgotten me, the LORD their God.” (Source: Hiligaynon Back Translation)
• English: “People will hear a noise on the barren hilltops.
  It will be the noise made by people weeping and pleading for God to be merciful to them.
  They will be admitting/saying that they have forgotten Yahweh their God,
  and that they turned away from behaving as God wanted them to.” (Source: Translation for Translators)