I live across the border (Croatia) and to be honest, in the EU4 time period, I'd be hard pressed to tell the difference between South German, North Italian, Slovene, Coastal Croatian cultures, just as I'd be hard pressed to tell the difference between South Hungarian, Northern Serbian and continental Croatian. Remember, in EU4, culture =/= language.
Historical Poland and the Balkans share a common feature - everybody else acted like they own the place and had no issues razing everything to the ground if it pleased the emperors in <insert the name of a very important capital here>. So apologies if we are a bit twitchy about religious/cultural/economic/political identities in the last two centuries century, but
we were busy from not getting wiped out in the EU4 timeframe.
If Austrians have a culture, so should Slovenes. So what if it's one or two provinces? It literally takes 30 seconds to type in a new name into the game database and add it to a culture group.
To be fair, this also raises another question which is culture group. I'd say the best way would be to have certain cultures be accepted in multiple culture groups. Slovene is case in point, it should be "accepted" by German, Italian and South Slavic culture groups at the same time. Except if my Slovenian friends object to this
If I'd have to choose, I'd say South Slavic would be the logical choice.
As mentioned before, EU4 culture is not the same as language. The Balkans, believe it or not, in the EU4 timeframe were a multi-ethnic community with significant influx from other parts of Europe. Unlike in "developed very important Western European nations", people in these parts were forced to speak several languages, at the minimum the language of their own "tribe" and that of their overlords. Which, for a typical Croatian (noble), meant speaking Latin, German, Hungarian, Croatian and Italian, at the very least. This also meant that culture was a mix of most european cultures, plus that of the orient (Turkish and Persian at minimum) coming from the Orient.
To conclude, as everyone was so busy razing everything every 10 years, yeah, the EU4 development level of the Balkans is about right. That doesn't mean that people living here in that period don't deserve the same cultural recognition as the most developed ones, especially since technically it was more vibrant and diverse (and still is) than we get credit for.
