Zephaniah says the LORD will change the peoples to “a pure speech.” The stated purpose is that they may all call upon the name of the LORD and serve him with one accord (Zephaniah 3:9).
In Zephaniah 3:8–10, Yahweh tells his people to wait for a coming day when he will act decisively. That day includes the gathering of nations and kingdoms for a sweeping, earth-wide reckoning described in intense, consuming imagery. These verses come near the end of Zephaniah’s closing movement in chapter 3, after sharp criticism of Jerusalem’s leaders and religious life (3:1–7). Verse 8 functions like a turning point: after describing repeated refusal to accept correction, Yahweh announces a set time to act.
These verses present a sequence: a time of waiting, a day when Yahweh gathers the nations for severe judgment, and then a “then” in which peoples are changed so they can call on Yahweh together and serve in unity (Stage A textual claims).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Zephaniah 3:9, then read the surrounding section so the answer stays connected to the passage's flow.
9For then I will purify the lips of the peoples, that they may all call on the name of Yahweh, to serve him shoulder to shoulder.