It looks like there are moment.js plugins for Hijri and Jalali calenders: Moment.js | Docs
To see how easy it’s to use, I cloned the Hijri plugin into vendor folder and changed the dates formats in client.ar.yml according to the plugin documentation and I got it to display Hijri dates in a few places:
(July 2017 is in 1438 Hijri year)
I’m guessing it didn’t display Hijri dates in all places because those were using different core methods for displaying dates and might need to be tweaked a bit to display Hijri dates correctly.
Overall I think Hijri support should be done in a plugin and in a new separate Arabic locale to let people choose between Hijri or Gregorian.