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