Karlie here from the Shopify Support Team! If you're using the Debut theme then you should be able to change your theme language from within your theme settings. If you don't have Arabic listed in your theme language settings, then you might need to simply update your theme first. You can check out this guide to see how you can update your theme. To change your theme language, you can follow our step by step guide, here.
If you're having difficulty with your language formatting correctly, then be sure to reach out to our team via email, chat or callback here. Our theme team can dig into a Shopify-supported theme's coding to make sure that the language is formatted correctly.
If you're finding that changing your theme language isn't working out for you, then you could always look into using an app to translate your storefront for you. Specifically I would recommend checking out the apps; Translate&Currency, Translate My Store, or Multi Lingo.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!