Despite a decisive victory for Britain in north America, they still allowed the French and natives (to a lesser degree) to keep their own language, religion and legal system. What motivations were there for this appeasement? Why not try to assimilate them?
What's your definition of lenient? The treaty of Paris was the largest transfer of land in history.
I am not very qualified to answer this question, so hopefully someone can build up onto my response.
I dont think they believed they were strong enough to pull it off. Despite its low population, New France was heavily militarized and the natives and French Canadians combined were a lethal guerilla force if rubbed the wrong way.
If that is the case, it was likely due to their similar ordeal with Acadians, which turned out to be something of well, a shitshow.
Remember that forced assimilation was also the intended policy with the Acadians, who actually lived in communities within short distances of English ones. Attempting to force Acadians to convert (religion seems to have a bigger deal that language back then) had proved to be a massive fiasco, resulting in intense ethnic conflict, even if most Acadians seem to have desired peace over open conflict. Deporting the Acadians, (although initiated by the nova Scotia colonial government) was followed through by the British, which resulted in massive Acadian casualties and likely a perceived huge spenditure of resources, for very little reward.
And given that New France was much larger, very religious, had a deeply militaristic culture, and had been fighting/threatening Britains hold in North America effectively up until and during the 7 years war for more than a century, it seems obvious why the British might hesitate to impose impunity on these people.
Sources: history class and Heroes of the Acadian resistance by Dianne Marshall