Zephaniah 3:9
"Then I will purify the lips of the peoples, that all of them may call on the name of the LORD and serve him shoulder to shoulder.
The pure language, שפה ברורה saphah berurah, may here mean the form of religious worship. They had been before idolaters: now God promises to restore his pure worship among them. The word has certainly this meaning in Psalm 81:6; where, as God is the speaker, the words should not be rendered, "I heard a language which I understood not," but, "I heard a religious confession, which I approved not." See Isaiah 19:18; Hosea 14:3; and see Joel 2:28, where a similar promise is found.
Clarke
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God turns to the nations a pure lip, by purifying their sinful lips, i.e., He converts them, that they may be able to call upon Him with pure lips. Lip does not stand for language, but is mentioned as the organ of speech, by which a man expresses the thoughts of his heart, so that purity of the lips involves or presupposes the purification of the heart. The lips are defiled by the names of the idols whom they have invoked (cf. Hosea 2:19; Psalm 16:4).
Keil and Delitzsch Biblical Commentary on the Old Testament
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3:9 A pure language - I will give them a pure way of worshipping me, the issue of a pure heart.
Wesley