Drupal website only in Arabic on English installation
First you'll need the i18n module to enable your drupal website for a multilingual setup. It should be no affair to have Arabic as the preliminary language, though you may think about having English as the default language due to some string issues. If you want a website with right-to-left text direction, you'll need a theme which supports this.

A good start to see what's also important can be found in this comprehensive tutorial.