Asking due to some research I'm doing for authenticity in a project of mine. As I understood it, the vast majority of french trade and occupation in general primarily took place in the early 1700's and diminished in the later half after the French and Indian war. Since the setting of this film takes place significantly later than these events, I guess I'm wondering what level of historical accuracy the french party depicted have, if at all. Appreciate the help.
The premise of your question is flawed. While in The Revenant the "bad guys" are indeed Francophones operating in the Missouri River delta, they are not intended to be French nationals (despite their leader, Toussaint, being played by a French actor). They're supposed to be French Canadians (to this day, about 20% of Canada speaks French--if this is hard to wrap your head around, just think of Celine Dion).
After the British victory in the Seven Years War and the expulsion of the French-speaking Acadiens from what is now Canada's maritime provinces, the Francophones in Lower Canada (now Quebec) were left more or less intact, and continued to supply the majority of the labour to Canada's fur trade. After the war of 1812, the US government prohibited Canadian trappers from crossing the Northern border into the US. However Missouri was admitted to the Union as a state in 1821, and much of the remainder of the former Missouri Territory existed for some time as unincorporated land, making southward pushes into what is now South Dakota and beyond not only possible for French Canadian trappers during the 1823 era depicted in The Revenant, but legal too.
Long before 1823, much of what is now Canada was incorporated as Rupert's Land, and directly managed by the Hudson's Bay Company (which continues to operate, now as the owners of Sak's Fifth Avenue and many other department stores), which had a monopoly on the fur trade in the area. The role of HBC (and the related North West Company, which was established as a competitor headquartered in Montreal, but later merged into the same business) in the westward colonial expansion in North America, and the settlement of the Northern frontier of Canada, really can't be overstated. Even Boise Idaho was originally set up as Fort Boise, an outpost owned and operated by HBC for trade in the west.
So yes, it's perfectly reasonable to see French spoken in the region and era of The Revenant, as long as you don't forget about French Canada.
One important note: Arguably the most grievous historical inaccuracy in The Revenant is not the presence of the Courreurs De Bois, but the depiction of their behaviour. Especially compared to their Anglophone counterparts, French Canadian traders were downright civil in the midwest, freely trading, learning indigenous languages, and intermarrying with the First Nations who owned the land. They settled much of what is now Manitoba and Saskatchewan as Métis, a hybrid franco-indigenous culture that continues to exist today. While rapes and slaughters did no doubt take place, they paled in comparison to the systemic genocides that the US and British governments organized during this era. It's very Hollywood to find a foreigner to make into a villain, and let the American be the civilized hero.
I don't have sources to give you because this is all super introductory Canadian history. Any errors are my fault and likely minor, but you shouldn't cite this post in anything academic. Here's a bit more on why Toussaint had to be played by a Frenchman, though--The leading actor(s) of Quebec cinema didn't want the part: https://nationalpost.com/entertainment/movies/canadian-actor-says-the-revenant-completely-stupid-for-portrayal-of-french-canadians-as-murderous-rapists
French Canadian fur traders were very common in western north America, often outnumbering their English speaking counterparts by a significant margin (1, 2). Many French Canadians established themselves as experts in the fur trade as far back as the 1600's, where many worked as Courer Du Bois (independent contractors procuring furs for trade). By the 18th Century, most were hired by the Hudson's Bay Company. This company was a British crown corporation who monopolized the fur trade (and other sorts of commerce) along all of the major rivers and tributaries running in to the Hudson's Bay. This is actually the reason why the Canadian / US border is on the 49th parallel (3).
When John Jacob Astor created the American Fur Company in 1808, he did so to challenge the Hudson's Bay Company dominance in what was then the Columbia River basin (jointly claimed by Britain and the United States). Another Hudson's Bay Company Rival was the North West Company (based out of Montreal). The Northwest Company traded as far south as Wyoming as well. Jacques La Ramee (whom Laramie, Wyoming is named after) moved to the area as a trader for the North West Company, and acted as an arbiter between them and the Natives of the area (4).
By the 1820's French Canadian fur traders far outnumbered their English speaking counterparts in the North American West (4). Back in 1820's Wyoming, it can be safe to conclude that as a fur trader, you would very likely be working alongside, and against, French Canadian fur traders and their Metis counterparts.
I will also take this opportunity to throw in a shameless plug: a heritage video about the voyageurs:
https://www.youtube.com/watch?v=M_YsuqkkBa8
I posted a reply to a similar question a while back and I'll repost a portion of it here. Essentially, the word "French" doesn't fit in this case.
One of the parts of "The Revenant" that was hardest for me to believe was the depiction of the "French" fur traders as the pillaging, raping group of foreigners and the Americans as somehow different. At the time and place, Montana, North/South Dakota, any French speakers who were there would have been essentially indigenous, Metis families usually in Mixed communities with Saulteaux, Assiniboine, or Cree, who made up the Nehiyaw-pwaat, or Iron Confederacy.
As to the Americans, they had a reputation for violence going back a looooong ways, at least into the 18th century, as variations on the name "long knives" that we use in Cree "kihci-mohkoman" (and I think Dakota as well - and yes, I checked - Isaå Taåka - long knives) go back long before the cavalry of the late American-Indian wars all the way back to at least the war of 1812.
One of the things that often gets skewed in historical retellings is the extent to which communities were separate. When the American/Canadian government moved into the prairies, they never knew how to deal with the Metis. In the states in some places they were called "White" and just treated as settlers, other places they were called "Canadian Indians" and pushed back over the border, while in still others they were called Cree or Chippewa, and given reservations, blood quantum, and all the rest. In Canada as well the government had a need to categorize everyone, and I think this reflects views at the time. As a result, fur traders often get depicted in stories as being "white", when in fact it was much more complicated. For example there's a really interesting write-up my mother's great-great-grandma's family, the Twatt family, here describing their development as a Cree band, who at times went by Orkney Scots names, at other times by Cree names, who were all fluently multilingual, who often had European educations, but who married, traded, and supported extended families as Cree, and who later on were signatories of some of the number treaties, as well as many of them taking scrip as Metis. These were people who were part of the local landscape, yet whose wives often wore long black dresses, and themselves tended to wear European hats, and who at home were often comfortable speaking French, Michif, and Cree, while equally comfortable speaking Dene, Saulteaux, Sioux, English and Gaelic with others.
This confusion is really common - people think of Mackenzie and his men as being a Scot with Frenchmen, but the most "French" of those Frenchmen had one Canadien grandfather, and they were able to speak with the Athapascan related groups (at least rudimentarily) to within fifty miles of the Pacific ocean.
I just stared watching the series Frontier myself, and so far, it's pretty out there in this regard. It claims to be set in the late 18th century. At this time the Cree were already in alliance with the Assiniboine and controlled much of the fur trade. The HBC was largely run by Scots from the Orkneys, with little or no "British" military presence. Almost everything about the show feels off, from the languages spoken, the accents, the military presence, the random killings, the lack of dancing, the music, even the views of the conflicts. I need to stop or I'll keep on and on, but there's so much to talk about. I'll probably still watch the rest of the first season though!