You can set your store's languages settings easily, by going to Langauge & Region through the Store Settings:

Then, click on Manage Language as follows:

To add a new language click on Add New Language:

You will be directed to this page to select a language from the Available Languages field highlighted in the following screenshot, the rest of the fields will be filled automatically and you will have the option to change the image and language name, then click on the Save button:

The new language has been successfully added! Going back to your store's languages, you will be able to disable, delete or edit any of your store's languages as follows:

Through the Language and Region settings, you can set the store default language from here:

The dashboard (Backend language) can only be English or Arabic, feel free to set the one you prefer as the default one.