How do I add the Arab League flag as language?
You can't add a flag as a language, if you can add a new language go to YOUR_SITE/admin/config/regional/language/add choose the new language if your language is not listed you can add a Custom language (See at the bottom of the page the collapsed option).

And if you want to use a flag for your language you can install the Language Icons module. Then go to the folder (in your server) YOUR_SITE/sites/all/modules/languageicons/flags and add the flag for your new language, you need to use as a name YOUR_LANGUAGE_CODE.png