If you want to display your windows 7 in Arabic, you should have at least windows 7 professional.
In Ultimate version, Arabic and many other languages are available and simply proposed as windows update.
If you have Windows 7 professional, you can manually add the package you want.
You can use a third part program to install and change the display language.
For example, Vistalizator is a tiny program that allows you to install a package and change your display language easily.