Actually there is no error in geography here. The setting of the town is in the Eastern Townships of Quebec, which was heavily settled by English-speaking refugees (the Loyalists fleeing the American Revolution). Many of the towns have English names (Drummondville, Lennoxville, Stanhope). The show was filmed in the town of Coaticook very close to the US border.
Judging from the flag in the mayor's office, Maplewood takes place in Québec. But throughout the series, the only francophone element is the character Mr. Deschanes's name.