In this play, mastery of the English language is a matter of national pride. No big surprise there, right? After all, Shakespeare is the guy responsible for putting English on the map. (No offense, Chaucer.) That's why Merry Wives of Windsor is full of the kind of clever word-play, innuendo, and snazzy banter that celebrates the potential of the English language. At the same time, the play also goes out of its way to mock characters (especially foreigners and members of the lower class) for butchering the queen's English. At the end of the day, Shakespeare wants to show us that English defines England.
Merry Wives is written mostly in prose (as opposed to verse) because it's a play about ordinary middle-class English life—not the lives of some poetry-spouting members of the royal court.
Shakespeare's Britain was a nation made up of English, Scottish, Welsh, and Irish speakers all from a wide range of socio-economic backgrounds. So, when our favorite playwright shows us a wide range of dialects, accents, and speech styles in this play, he's giving us a glimpse of the diversity of the English language.