If, in the 14th century, you travelled from the southern tip of Italy to the North Sea coast of Zeeland, you would encounter a significantly different dialect every few days' ride (on horseback, some 50-100 miles, if you were pushing it). Each of these differences would be about as great as the contemporary difference between American and British English. Travel a week or so in this fashion, and you will find mutually incomprehensible languages (or dialects) at either end of your journey. People who travelled regularly, and this was a few percent of the population, would have to be conversants in several languages or dialects in order to survive, although they could always speak Latin .if they encountered a priest or other educated person.