Make sure you leave the Default and then add Arabic too:
Quite possible that some apps choose to only show the Default. Would not be surprised if Illustrator is one of these apps. Are you using Illustrator with Arabic menus? Then it is an Illustrator bug not to display the appropriate entry. I just had a look myself, and Adobe only offers me English with Arabic support (يدعم العربيق) as ‘install language’:
I assume the menu language is still English? Then it is correct of Illustrator to show the Default entry. Workaround: you can write the Arabic word (or both English and Arabic next to each other) into the Default field.