Writers often use repetition to emphasize an idea. The author of Psalm 47 highly stresses that all nations (cf. Ps. 47:1) must sing praises to God by repeating the same command five times in rapid succession:
Sing praises to God, sing praises: sing praises unto our King, sing praises. For God is the King of all the earth: sing ye praises with understanding (Ps. 47:6-7).
Furthermore, two references to God as King in these verses and the use of for at the beginning of verse 7 emphasize that they must sing praises to Him because He is the King of all the earth.
David writes of the day when God will receive such universal worship:
All the kings of the earth shall praise thee, O LORD, when they hear the words of thy mouth. Yea, they shall sing in the ways of the LORD: for great is the glory of the LORD (Ps. 138:4-5).
He teaches here that all the supreme leaders of all the nations will one day sing praises to God because of the greatness of His glory.
Correlating the teaching of these Psalms with Matthew 28:18-20 points to a vital truth: We are to disciple all nations to sing praise to God, the King of all the earth!