Zephaniah 3:9
For 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. For 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.
The promise comes after the gathering of nations and the assembling of kingdoms to receive poured-out indignation and fierce anger, when all the earth is devoured by the fire of jealousy. After that judgment God turns to the peoples a pure speech so that all call upon the divine name and serve with united consent. These are conditions to exist in the future, given as warning.
That pure speech belongs to a people cleansed from shame and transgression. Those who rejoice in pride are taken away and haughtiness is ended, leaving an afflicted and poor people who trust in the divine name. They are a remnant that does no wrong and speaks no lies, with no deceitful tongue found in their mouth, feeding and lying down with none to make them afraid.
It points to a coming time when the barrier of sin between peoples will be gone and they will commune together without differences of language hindering them. The saved will share one language with no barriers to fellowship with one another. The same promise applies now to the need for clean lips and pure breath in those who preach and press moral reform. To indulge in tobacco while urging temperance upon others is wrongdoing against God that destroys moral influence.