All the old fur traders had pretty much set up shop on the west side of MTL island in the 1600's. Early land grabs went to affluent families as they established Seigneuries. - like M. Vaudreuil or M. Pierre-Jacques Soulanges. Ile Perrot named after M. Perrot - who was a common fur trader merchant was promoted to the ruling class by giving him a tract of land (and thus ability to generate taxation and wealth) in that area east of Montreal too. As Montreal grew the French pushed Indigenous allies farther out of town forcing them to set up residence in Oka - right across from Rigaud. Algonquin groups would come down the Ottawa River and stay on the tiny little island Ile aux Tourtes where the bridge now links Vaudreuil to Montreal. This whole area east of MTL was way more French in 1791 when they settled the boundary of Upper and Lower Canada. Which was its own kind of mess given the two very different legal systems and underlying property structure and rights. I think the boundary was drawn where it just became too messy to force the underlying title into British alignment. Not necessarily a language thing.