Culture change

[unmerged(23302)]

on culture change, this question has probably been asked a million times but.... i forget, will a county change to your rulers culture eventually? I started out as the count of Verdun which is German culture with a French count but I have now for some years been a subject of the French crown. Ive made sure that my rulers are always French, i would love to have my whole realm French which has become the Duke of Upper and Lower Lorraine, thanks to my very kind French king who since ive been a strong supporter of gave me the titles after i did battle in the area, very rewarding. i usually separate from my rulers. Anyway back to the culture change thing, will i have to do it manually? If so how?

 

[Count of Reval]

Province's culture can change by an event. So (unless altering save-file) you can't change a culture manually, but you can increase your chances of having the conversion event (by building special improvements into target and source provinces etc).
If the event happens, then particular province acquires a culture of the highest tier ruler. So in your case, your German provinces would turn into French culture as long as your liege is French. Your own (a vassal's) culture isn't important.