How to write Arabic and other RTL languages in Scribus?
As far as I know the support is still in development.
Keep your eye on this repo: https://github.com/andreas-vox/ScribusCTL
This is where the project to get RTL languages to work in Scribus will happen.
The HOST work on Arabic (https://github.com/HOST-Oman/scribus) has been merged into Scribus!
It will be available in the upcoming development version 1.5.3 and in the next 1.6 stable version.