In that day

"At that time"

this song will be sung in the land of Judah

This can be stated in active form. AT: "the people in the land of Judah will sing this song" (See: rc://en/ta/man/translate/figs-activepassive)

This refers to the city of Jerusalem.

God's power to protect and save his people is spoken of as if his salvation were walls around a city. (See: rc://en/ta/man/translate/figs-metaphor)

Here "nation" represents the people. AT: "the righteous and faithful people" (See: rc://en/ta/man/translate/figs-metonymy)