Following are a number of back-translations as well as a sample translation for translators of Jeremiah 30:9:

• Kupsabiny: “But they shall serve me who is their God,
  and do (it) for the king who comes from the house of David
  whom I will appoint for them.” (Source: Kupsabiny Back Translation)
• Hiligaynon: “They will- now -serve me, the LORD their God, and their king which-is the descendant of David whom I will-send for them.” (Source: Hiligaynon Back Translation)
• English: “My people will again serve me, Yahweh, their God,
  and they will serve a king who is a descendant of King David;
  they will serve the king whom I will appoint for them.” (Source: Translation for Translators)