The Atoning Messiah is expounding his future mission as the Triumphant Messiah, which takes place at the end of times when the judgment of the nations will take place.
Reference: The language of this passage echoes Revelation:
Revelation 7:9
9 After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
The quadruple occurrence of people, kindreds, nations, and tongues certain owes that particular collection of descriptions to Revelation. The context of judgment also fits this particular verse in Revelation. While the verse is not a direct citation, the vocabulary of the verse served as the model for Joseph’s translation of the meaning that was on the plates.