RobH_en_tn/isa/49/12.md

the land of Sinim

This probably is a region in the south of Egypt near Aswan. (See: rc://en/ta/man/translate/translate-names)

Sing, heavens and be joyful, earth; break into singing, you mountains!

Isaiah turns his attention from the people of Israel and speaks to the heavens, earth, and mountains as if they were people. (See: rc://en/ta/man/translate/figs-apostrophe)