Well, it matters far less what the dialect is called than to acknowledge that it exists. "Middle English" is hardly a unified tongue outside of London or so (and even that had it's own distinct peculiarities, iirc.) It was exactly in our period, 13th century and on, that the political and trade pressures began to consolidate these little backwaters into what we now know as "countries", and individual languages.
It could be a simple rule that, except for some significant exceptions, any peasant tongue is only good "around here". English is currently broken into "Anglo-Saxon English", Welsh, the misc. Viking influence up North near York (Yorvik), some dialect spoken out SW toward Land's End, and a few more I can't remember, etc.
The difference between "being intelligible" is similar to the difference between "newscaster English" and inner-city gang-land speak, yo, so I don' 'spect you be frontin', o' we be scufflin' d'rectly, yo?. The two can be understood at a basic level, but are hardly the same for complex or accurate communication.
It doesn't require becoming a medieval linguistic scholar, merely finding a good map that shows the breakdown (here's a good political one, tho' not so much for all langs: atlas-games.com/pdf_storage/europemap.pdf)
Every major cultural/political area had its own language, sometimes more than one. The nobles of England spoke Norman, from Normandy, where William t.C. and his buddies came from. Southern France speaks Provencal, Northern France speaks Frankish. Parisian is a distinct island, Norman is another xenophobic cultural bastion, and so is the tiny, independent corner of Gascony (think "Texas"), and so on.
Just to the south, there is "Spain", which could, again, roughly be broken down into the kingdoms listed- Portuguese, Leonese, Castillian, Aragonese, and, of course, the Basque and "Arabic" (which each present their own problems).
But if you want to make life easy and ignore the whole thing, it's hardly a dealbreaker. I think it adds to the flavour to the game as a whole, but I don't deny I've found my views in a minority before.