It will come about

This phrase is used here to mark an important event that will happen. If your language has a way for doing this, you could consider using it here.

the glory of Jacob will become thin, and the fatness of his flesh will become lean

Here "Jacob" refers to the kingdom of Israel. Israel would no longer be glorious. Rather it would be weak and poor. (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-metaphor)