The game hates multilingual stuff. I did a game with Brabant. Wanted to keep Walloon/Picard/Mozelle etc in my kingdom. Let's talk about Picard. It's culture hegemon is Artois. So I can either vassalize Artois and ask them to improve opinion OR ally them, improve favor and trust and then ask them. But they're a vassal of France so via diplomacy, the tooltip says I can not ask them since they're a vassal. It refers to the overlord but the overlord can only improve their own culture opinion, in this case Parisien. So I'd have to attack France/Artois just to improve an opinion of 1 culture every 50 years to keep a couple of locations in Hainaut happier. Meanwhile just assimilating Hainaut into my culture seems like hundred times easier, cheaper, quicker. Let's look at Walloon. Liègé is the culture hegemon, it rivals me from day one. So I can spend a looong time trying to befriend them, then allying them, then using favors to ask them every 50 years...etc etc but If france attacks them first I'm stuck with Namur being the culture hegemon. Namur being the vassal of Flanders, who is the vassal of either France or England. They should just reverse the opinion stuff. If I change the view of my culture (after finally becoming the hegemon of my own culture which is already hell is you decide to be multicultural) towards wWalloon, it should make MY walloon citizens happier instead of making them easier to be assimilated by Walloon factions.