If you set the default language of the board to Arabic a guest will still only see English questions.

This is caused by the line:

$this->question_lang = $user->data['user_lang']; 

It's grabbing the language from the profile of Anonymous, not the board's default language.
Also on the registration terms page, after choosing Arabic, the $user->data['user_lang'] isn't set to Arabic but stays English.

For the posting page (captcha) this could be easily solved by using $config['default_lang'] instead of $user->data['user_lang'].
But for the registration page you can now choose your language first... so then the user_lang should be overwritten I think.