[readingglasses]

You could also just convert any document in arabic/hebrew/persian etc from html/epub or any other format into PDF (page formatted for your screen, if you're half good at using calibre) and read it on a reader as pdf, as a last resort.
But you'll lose the ability to zoom font size without zooming the whole page, like you can with epub.
But maybe other readers allow you to change font size within a reflowable pdf. From what I've read, kobo doesn't allow the reflow of a reflowable pdf, it only presents the base page formatting as it's set in the pdf, unlike everyone else. But it's cheaper and simpler design than everyone else. So you may be stuck zooming and panning a pdf to read arabic, or else work at formatting them "just right" in calibre before transferring to kobo.

Or you could convert them to an image or series of images, in calibre or whatever, that the kobo (like others) supports: jpg, gif, bmp, png.

So it's not that you "can't" read arabic on kobo or other ereaders. It's just that you can't read it in epub version 2 as it exists on all readers today. But I haven't tested yet and maybe it can be done.