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).

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.

Preferred Locale vs Admin Locale