Middle French

choi

(1) "Middle French (French: moyen français) is a historical division of the French language that covers the period from (roughly) 1340 to 1611," citing dictionnaire du moyen français. Paris: Larousse, 1992, at page v.
en.wikipedia.org/wiki/Middle_French

I can not find any other source to support those years of demarcation, and it seems nothing dramatic happens in those two years to merit the demarcation.

(2) History of the French Language. In Official Languages of Canada. Official Languages and Bilingualism Institute (OLBI), University of Ottawa, undated
www.slmc.uottawa.ca/?q=french_history
("[section heading] Middle French (14th and 15th Centuries) and the Renaissance (16th Century): "Henry V married Catherine de Valois, the daughter of Charles VI, the King of France. * * * Whether it had been the Duke of Burgundy or the King of England who occupied the throne of France instead of Valois, both contenders spoke French")
(a) Hundred Years' War
en.wikipedia.org/wiki/Hundred_Years%27_War
(from 1337 to 1453 between the House of Plantagenet, rulers of the Kingdom of England, against the House of Valois, rulers of the Kingdom of France, for control of the latter kingdom)
(b) House of Valois
en.wikipedia.org/wiki/House_of_Valois
(was the royal house of France from 1328 to 1589; The Valois descended from Charles, Count of Valois (1270–1325), the fourth son of King Philip III of France (reigned 1270–1285); section 2 List of Valois kings of France, section 2.1 Valois (direct);
(i) Those French kings in Section 2.1 are what University of Ottawa alludes to in "instead of Valois."
(ii) I fail to find out who " Duke of Burgundy" was who contended the throne.
(iii) Valvois is a historic region, see the map in French Wikipedia,
Valois (région)
fr.wikipedia.org/wiki/Valois_%28r%C3%A9gion%29

, which "gave its name to the second line of the Capetian dynasty; it corresponds to the southeastern quarter of the modern département of Oise, with an adjacent portion of Aisne."
Valois. Encyclopaedia Britannica, undated.
www.britannica.com/EBchecked/topic/622375/Valois