Actually, French language was not used officially in any documents until the
time of king Francois I in the XVI th century. Before, it was only the
language of the people around Paris. Latin was the official language. People
spoke various dialects in each region, some of which still exist even now. It
was not until the French revolution that french was universally acknowledged
as the common language and local tongues only really faded after the
slaughters of the 1870 and first world wars.

So, if you want something close to actual history, you should have one
different language for every important town, several unpalatable dialects in
the countryside, and a "common" tongue of the elite deriving from an ancient

Horten the mage