Example pages: the "Ummah for Earth" sites, but I've also seen the issue in MENA sites

• Arabic version (broken)
• English version (works fine)

In the broken one you can see that the RTL direction is not set on the html element in the editor, despite the language being Arabic, and the sidebar renders on the right instead of the left with some missing editing options such as for the CarouselHeader block (see screenshots). 

It might be due to WPML since it seems to be happening in multilingual sites.

Beside breaking RTL, the same issue also caused certain editor labels to be translated into the post language while they should follow the editor language.