Albert Barnes Commentary Psalms 138:4

"All the kings of the earth shall give thee thanks, O Jehovah, For they have heard the words of thy mouth." — Psalms 138:4 (ASV)

All the kings of the earth shall praise thee ... - That is, kings, princes, and rulers shall learn the words of promise; shall be made acquainted with the words which you have graciously spoken, and with their fulfillment, and shall be led to praise you. This refers to a time, of which frequent prophetic mention is made in the Scriptures, when kings and rulers shall be converted to the true religion, and when they shall act an important part, by their example and influence, in maintaining and diffusing it. Compare Psalms 68:31-32; Isaiah 49:23.