Selecting Arabic language in Greeter results in English keyboard layout
... and Arabic isn't even available in GNOME's keyboard layout selector (in the "systray"). In other words, users must manually configure an Arabic keyboard layout in GNOME Settings.
In Tails 2.12 (the last 2.x release) this was fine; selecting Language = Arabic in the old Greeter results in Arabic keyboard layout. So this regression was introduced in the new Greeter, in Tails 3.0 (and its alphas/betas).