הַרְנִ֤ינוּ גוֹיִם֙ עַמּ֔וֹ כִּ֥י דַם־עֲבָדָ֖יו יִקּ֑וֹם וְנָקָם֙ יָשִׁ֣יב לְצָרָ֔יו וְכִפֶּ֥ר אַדְמָת֖וֹ עַמּֽוֹ׃ (פ)
Sing aloud, O ye nations, of His people; For He doth avenge the blood of His servants, And doth render vengeance to His adversaries, And doth make expiation for the land of His people.
Rashi on Deuteronomy
הרנינו גוים עמו SING ALOUD, O YE NATIONS, OF HIS PEOPLE — At that time (when I shall take vengeance on them) the nations will praise Israel (Sifrei Devarim 333:2): “See, how praiseworthy is this people, — that they have cleaved to the Holy One, blessed be He, amidst all the troubles which have passed over them, and have not forsaken Him, for they had constantly experienced His goodness and His excellence”.
Sforno on Deuteronomy
הרנינו גויים, “you who are the arrows of the Lord and His sword make the nations acclaim, sing aloud! So that they will realise that G’d is fair and does not commit any injustice. Compare Psalms 67,5 ישמחו וירננו לאומים כי תשפוט עמים מישור,”nations will exult and shout for joy, for You rule the peoples with equity”
Or HaChaim on Deuteronomy
הרנינו גוים עמו, "O nations, sing the praises of His people, etc." Moses means that when the time comes that G'd exacts retribution from the nations, from the enemies of the Jewish people, the other nations will praise the Israelites and jubilate and elevate them, as we know from Psalms 144,19: "Hail to the people who experience this."