There are no plans to split up the current locale system into locale and UI language in the near future. However, I agree, it would be a good thing to do.
Anyway, I think the current system can already be used to create region specific locales. We have client.en_US.yml and server.en_US.yml which override date/time formats, numbers, etc.
I’m sure something similar could be done for Arabic. It should even be possible to do this with a plugin.