Zephaniah 3:9 (ESV) … “For at that time I will change the speech of the peoples to a pure speech, that all of them may call upon the name of the Lord and serve him with one accord.”
God always intended that the people of Israel should be a blessing to all nations. It was because Israel failed to minister the truth and grace of God to the nations, that evil kingdoms rose up and went on the rampage. But God has His own agenda, and when Israel fails to live up to its spiritual responsibilities, He pursues His own program anyway.
God plans to cleanse the speech of the Gentiles in the endtimes: “Then will I turn to the people a pure language, that they may all call upon the name of the Lord, to serve him with one consent” (3:9). Some people think Zephaniah envisioned the nations speaking a common language during the millennium—possibly Hebrew.
In his vision the prophet saw the Gentiles “from beyond the rivers of Ethiopia [the Blue and White Niles]” (3:10). We can infer that Jews living as far away as Ethiopia will be brought—by the Ethiopians—to their rightful homeland in Israel and that the Gentile Ethiopians will thus be bringing a kind of offering to the Lord.
While the prophecy in Zephaniah 3:9–10 is millennial, there has been a provisional fulfillment of it in the outreach of the gospel to the nations of the earth during the present church age. The stubborn, deliberate, persistent rejection of Christ by the Jews, as recorded in the Gospels, the book of Acts, and several Epistles (see Romans 9–11), did not hinder God from pursuing His purpose of saving a host of Gentile people.
 Phillips, J. (2009). Exploring the Minor Prophets: An Expository Commentary (Zep 3:9–10). Kregel Publications; WORDsearch Corp.