The first part of this verse repeats verse 15.

I will put my words in his mouth: the literal equivalent in many languages will be natural and vivid; in English it is better to say something like “I will give him the message he is to deliver,” or “I will tell him what to say” (Good News Translation).

He shall speak to them: them is the people of Israel. So we may say “He will tell the people of Israel everything that I command” (similarly Good News Translation).

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .