Amazon announced support for Arabic language books on e-ink Kindles in the What's New release notes for 5.9.6 and then later removed it. The latest version (5.9.6.1) supports Arabic language books purchased from Amazon, such as this one:
هاري بوتر وحجر الفيلسوف: 1 (Harry Potter) (Arabic Edition) Kindle Edition
Arabic language books sold by Amazon are delived in KFX format and do not seem to have any noticeable page turn lag.