[Aqwis]

I wonder if Slovenia did really not switch before 1919. It was part of the same "country" as what are now Austria and the Czech Republic for hundreds for years before that, who according to the list switched in the 16th century.

It seems unlikely that what is now Slovenia would use a different calendar than the rest of the Habsburg Monarchy/Austrian Empire for such a long time, particularly since there was no subdivision of the Empire that directly corresponds to modern Slovenia.