No it isn't irrelevant, and it is no more a fantasy mod than vanilla or AGCEEP.
Culture has to be defined for the game in some way.
Language is only part of it, at least here. Then there are commonly held laws, race (or the perception of it) and traditional animosities to consider.
Occitan is still used today ... precisely my point, which is why the language spoken by the common people is no use to us as the yardstick by which we measure the culture of a province as defined in the parameters of EU2.
this is true, but your MOD and AGCEEP need to establish what levels of culture ( if you have many culture to spare ) exist. You cannot say that slavonic is justified to be used when you deal with craots and serbs who have by the way an different linguistic alphabet to each other, let alone a difference in religion.
as for languages, well dialects are languages , its only the government of the nation that seem to recognise or not , a language from a dialect. example, there was no Croatian , slovenian and moldovian language only as dialects, but since they gained nationality, their dialects are now languages. also, a further example, we know frisian is a language and culture but the dutch government say its a dialect. IMO there are not many dialects ( as per the greek terminology of dialect) that exist in the world today. So people who say , you cannot have this culture Because its a dialect are in error.
As for the game, I think that "wrong culture" needs to be more emphasized. eg
stab of + 1 , 2 and 3 ...........as per game now
stab of 0 and negatives .........double trouble........60% loss of manpower and taxes and double revolt risk
As you say we have the commands already in place to grant, remove or change province cultures.
I feel, certain strong nations ( depending on game trends) need to per culled in strength by using certain cultures, similar to what AGCEEP did with SPA.
France can be "culled" by introducing occitan till 1536, then these provinces change to french or FRA gain occitan culture