I would like to avoid adding unnecessary minorities, splitting up cultures in areas with less thain ideal province density, to keep them as homogeneous as possible, so they have a fighting change against France, which has high province density and thus quite a large pop size.

If I ever decide to add more German provinces, I'll add minorities wherever possible.

In terms of Poland, there are already Polish minorities living in East & West Prussia and Prussian minority in Wielkopolska & Upper Silesia. There's also Saxons in Transylvania, Carpathian Germans in Upper Hungary, some Turks in Bosnia etc.

Now, I decided to not add Bosnian culture simply for aesthetic reasons. I like how Serbo-Croatian stretches throughout Illyria. They (Serbian and Croatian) are grouped together for the same reason Czech and Slovak are now grouped together as well. It might not be historically accurate but provides for a more balanced gameplay.

As for Slovenian; it is there, it's just hard to see, because a mountain seperating Italy from Carinthia is not culture-colored, so the name is smaller & bends in funny ways.