Disclaimer: I am almost completely uninformed about other languages beyond assorted superficial details about a handful. That said, I would have expected Arabic to be of no greater relative difficulty than, say, Hindi, Persian, Hebrew, Mongolian, or such.

I mostly expected Mandarin, Cantonese, and Japanese to be of greatest relative difficulty, from what little I know about those languages, and would have expected Arabic (and many other languages) to be in the second category.

Arabic may be especially difficult (even as compared to Hebrew, a language I have studied much more) because the standard form of the language is so divergent from the day by day vernacular forms, as noted by a more informed participant who has kindly posted another comment.

The United States federal government language proficiency rating system attempts to identify equal levels of real-world functional proficiency for ratings of interpreters and the like. When I was last tested, the rating scale went from 0 (with a readily apparent meaning) to 5 (a rating in principle reserved for educated NATIVE speakers of the language) in the functional domains of speaking, listening, reading, and writing. I was quite proud of my 4+ rating in speaking and listening to Modern Standard Chinese, but I took quite a few years out of my life to get to that level, as have most westerners I have ever met who have done the same.

P.S. I just found, by browsing, that the Wikipedia articles about Chinese writing still need a lot of clean-up. Too bad.