John Wesley on Isaiah 19:18

In that day - After that time. In the times of the gospel. Five - A considerable number of their chief cities: a certain number being put for an uncertain. Speak - Profess the Jewish religion, agree with them in the same mind; which is fitly signified by speaking the same language. Swear - This implies the dedication, and yielding up of a person or thing to the Lord, by a solemn vow, or covenant. One - Not one of the five, but another city, the sixth city. As divers cities shall be converted and saved, so some other cities shall continue in their impenitency, and be destroyed.