Albert Barnes Commentary Psalms 68:32

"Sing unto God, ye kingdoms of the earth; Oh sing praises unto the Lord; Selah" — Psalms 68:32 (ASV)

Sing unto God, ye kingdoms of the earth—that is, you who acknowledge the true God, celebrate his praise. The psalmist sees the conversion of the world to God to be so certain an event that he calls on all nations to join in the song.