Following are a number of back-translations as well as a sample translation for translators of Jeremiah 6:28:

• Kupsabiny: “All these people are rebels and their heads are hard/stubborn.
  They have loved to go around backbiting,
  and their heads are hard like bronze and iron.
  They are corrupt and liked bribes.” (Source: Kupsabiny Back Translation)
• Hiligaynon: “All of them are rebellious and hard-headed, as hard as bronze and iron. What they do is making- others -bad and deceive (other).” (Source: Hiligaynon Back Translation)
• English: “You will find out that they are very stubborn rebels,
  they are always slandering others.
  Their inner beings are as hard as bronze or iron;
  they all continually deceive others.” (Source: Translation for Translators)