Hopefully not reporting this too soon, (writing this is what brings the bugs out) but I'd like to say that RTL input seems to work well out of the box.
Rendering the saved page needs to have a 'dir' attribute or CSS 'direction' rule (totally expected) but the Volto text widgets are functioning with Arabic.
At least our english-speaking cut-and-paste tests are working, but it's likely that native language users will have a similar positive experience.
This question came up at the conference, so I just wanted to at least answer on progress.
Cool, I was just thinking about that the other day. Feel free to open an issue for making sure the dir and direction get set correctly based on the current language.