The Hebrew literally says that the people would serve with one shoulder.
This is a reference to the unity of the people whose speech has been transformed. In contrast to the problematic unity at the time of the Tower of Babel, this promised unity will not be an ethnic and linguistic unity but rather a unity of all humanity in relationship with God. Thus Zephaniah does not envision a return to a pristine primeval situation but rather he sees the redemption of the post-Tower of Babel humanity.
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9 “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.