I had never seen this study that OP is using but it seems all the categories map to this essay, Japanese being the most difficult and Spanish being the easiest.

I had never seen this study that OP is using but it seems all the categories map to this essay, Japanese being the most difficult and Spanish being the easiest. https://www.nsa.gov/news-features/declassified-documents/cryptologic-spectrum/assets/files/foreign_language.pdf https://www.nsa.gov/news-features/declassified-documents/cry... The essay specifically mentions the writing systems as a reason for the rankings