Hi,
You will have to create an 'en' page for english, then move all pages under this page (assign the parent page to the 'en' page).
You will need to install an arabic language pack.
You will need to create a root language page for your arabic language, using the 2 first letters of your language pack.
You will need to copy english pages under the arabic language root page, then translate content on these new pages.
Finally, to apply Right To Left direction to text, you will need to create an arabic css file with the "rtl" css attributes, and link this file dynamically using our skin localization approach to link this file only when the arabic language is the current language (there are some threads on this forum discussing this approach).