Contradiction with Genesis 11:7-9
This passage describes the confusion of languages at Babel, which contrasts with Zephaniah 3:9's depiction of a united language for worship.
Genesis 11:7-9: Go to, let us go down, and there confound their language, that they may not understand one another's speech.
Contradiction with 1 Corinthians 14:33
States that God is not the author of confusion, yet the diversity of languages from Babel suggests otherwise, opposing the unified language in Zephaniah 3:9.
1 Corinthians 14:33: For God is not [the author] of confusion, but of peace, as in all churches of the saints. [confusion: Gr. tumult, or, unquietness]
Contradiction with Jeremiah 12:17
Talks about the destruction of nations that don't obey God, whereas Zephaniah 3:9 speaks of nations uniting in pure language to serve God.
Jeremiah 12:17: But if they will not obey, I will utterly pluck up and destroy that nation, saith the LORD.