Louy Alakkad

This plugin gives you the ability to run the site in a language (Arabic for example) and use the admin panel in another (like English).
Just go to Settings » General and choose your language.

Admin Locale

This plugin allows you to change the language of the admin panel without changing the whole site language.

Sam Bull

Sets the site locale dynamically according to visitor preference. 
This preference comes from the user’s browser.
To install all available translations click the ‘Install All’ button in Settings->General. 
To install individual locales, select the site language and save changes for each locale you want to install.
Technical details: 
 Will parse the Accept-Language HTTP header and find the best matching language for the user from the translations available on the site.

Preferred Locale

Select site locale/language according to user preference (set in their browser).

Admin Locale vs Preferred Locale