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.

JS Morisset

Add a user locale drop-down menu item in the WordPress admin back-end admin and front-end toolbar menus.
Perfect for anyone that needs to switch languages quickly and easily – allows logged-in users to change their preferred locale / language setting right from the toolbar menu (instead of having to update their WordPress user profile page).
The default WordPress behavior is to apply the user locale preference to the admin back-end only – this plugin extends the user locale / language preference to the front-end webpage as well.
There are no plugin settings – simply install and activate the plugin.
<h4>Do you use the Polylang plugin?</h4>
If the Polylang plugin is active, the user locale menu will automatically use the correct Polylang language URLs for the current webpage.

JSM User Locale Selector

Hi, I think this plugin is really useful, stable and reliable.
I've been recommending it in the BuddyPress forum and also now in my own translation helper plugin which I designed to work with this: [link removed]
So excellent, 5 stars.

Really useful

A simple but effective plugin. The convenience of a quick admin language switcher saves precious seconds. Thanks.

Effective

Add a quick and easy user locale / language selector in the WordPress admin back-end and front-end toolbar menus.

Admin Locale vs JSM User Locale Selector