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

Provides a convenient locale / language / region selector in the WordPress admin toolbar.
Perfect for anyone that needs to switch languages quickly and easily:
Allows logged-in users to easily change their preferred locale / language / region setting right from the toolbar menu, instead of having to modify their user profile settings.

The default behavior of WordPress is to apply the user locale preference to the admin back-end only – the WPSSO User Locale Selector add-on can optionally extend the locale preference to the front-end webpage as well.
<h3>Do you use the Polylang plugin?</h3>
If the Polylang plugin is detected, the locale selector will automatically use the correct Polylang language URLs.
<h3>WPSSO Core Required</h3>
WPSSO User Locale Selector is an add-on for the <a href="https://wordpress.org/plugins/wpsso/" rel="ugc">WPSSO Core plugin</a>, which creates extensive and complete structured data to present your content at its best for social sites and search results – no matter how URLs are shared, reshared, messaged, posted, embedded, or crawled.

WPSSO User Locale Selector

Quick and easy locale / language / region selector for the WordPress admin toolbar.

Admin Locale vs WPSSO User Locale Selector