@GemmaWestwood
I am going to claim ignorance on the use of non-English languages, so I am not sure if this is helpful or not.
Switching the direction of the text does not require editing HTML. From the Rich Content Editor menu, go to the Format menu, choose Direction, and there you can set left to right and right to left.
This adds a dir="rtl" attribute to the paragraph containing the cursor. If you hit enter to start a new paragraph, it remembers the direction setting. This allows you to have both left to right and right to left text on the same page without editing HTML.
The menu does not have an option to add a language attribute, though. For that, you can edit the HTML and add lang="ar" to the paragraph. As with the dir attribute, the lang attribute is copied when you start a new paragraph.
I do not think that will fix the issue with the immersive reader, but I am not an expert there. I went to Google Translate and created some Arabic text. Then I copy/pasted it into a paragraph with dir="rtl" and lang="ar" attributes. When I asked the immersive reader to read it, it skipped it completely.