Give them this charge for their masters sounds rather strange to native speakers of English. It would be more natural to say “Give their kings this command” (see Good News Translation). In the context their masters refers to the kings of these states.

The LORD of hosts: See 2.19.

This is what you shall say to your masters essentially repeats the information at the beginning of the verse. In Die Bibel im heutigen Deutsch the entire verse is expressed more briefly:

• The God of Israel, the Lord of the entire world, commands you to deliver this message to your kings.

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on Jeremiah. (UBS Helps for Translators). New York: UBS, 2003. For this and other handbooks for translators see here .