When using a custom font with Arabic language, both the device and the simulator displays empty blocks instead of the actual characters.
This does not seem to have anything to do with right-to-left languages since it works perfectly with Hebrew.
I have attached a sample project with three language resources: eng, ara and heb.
To reproduce on actual device, export the app to device and sideload it onto actual device.
Change the language in settings to either English, Hebrew or Arabic, and return to the watch face.
Observe that the watch face does not display the characters correctly (empty blocks) for Arabic.
To reproduce in simulator, run the watch face
Change the language in the settings menu of the simulator to either English, Hebrew or Arabic, and return to the watch face.
Observe that the watch face does not display the characters correctly (empty blocks) for Arabic.