I would agree with using German, simply based on how they would have referred to themselves. Carniola was inhabited by Slavic people, but Istria had a more complex linguistic make-up with heavy Roman/Venetian influence in the coastal areas and beginning Slavic penetration inland. They were not part of the kingdom of Italy but first belonged to Bavaria and then Carinthia until they were separated out in 1040. The Markgrafen at this time also didn't really full-time live there and that got to the point that the later ones (Andechs-Meranien) barely ever set foot there. So for our time frame I would see the title as a Holy Roman Empire title bestowed on Germanic nobility without much connection to the people living in the land they ruled.