
Using the Sling Resource Merger in AEM, overlay the /apps/granite/ui/content/userproperties/preferences/form/items/language/items/* node.
 
Make sure you the overlayed language node as the sling:orderBefore property, so that the configuration is placed in the same area as the original configuration as expected:

 
 
Making sure that the ar language options have been added under the language/items resource.

 
 
I've created a package that you can install in your AEM local, give it a try. 
https://drive.google.com/file/d/1_vaICR3ntEyRMU7SLq8MEBtD7GOvB7IO/view?usp=sharing