How to add arabic language in Apple TV?

Change the language or region format on Apple TV - Apple Support

if arabic is missing then you can suggest they add it by using

Feedback - Apple TV - Apple

you can't manually add new languages to the device

For content languages, you have to choose from what is offered in the stream, which may or may not include the language of your choice. Users can’t add to that. Availability may differ by country or region, and by title. Change subtitles or languages for videos in the Apple TV app or iTunes Store - Apple Support