In the psalm’s  context, this diversity of the nations and the peoples is not limited simply to an evangelistic program. It is particularly related, rather, to the praise of God, or worship; ethnic identity must receive a liturgical as well as an evangelistic form, for it is properly in worship that a people’s culture is centered and sanctified. “Praise the Lord, all you nations” is a command weighted with immense significance for a people’s poetic language, music, architecture, art, and other cultural expressions.
And why do the nations (ethnoi) and the peoples praise the Lord? “For His mercy (eleos) is confirmed upon us, and the truth of the Lord abides forever.” When St. Paul quotes the first half of our psalm in Romans 15:11, it is in support of his large argument “that the Gentiles (ethnoi) might glorify God for His mercy (eleos)” (15:9).
— Patrick Henry Reardon, Christ in the Psalms, 233-234