unclego

Lets you have your backend administration panel in english or any installed language, even if the rest of your blog is translated into another language. Language preferences can be set per user basis in user profile screen.
Tested on MultiSite.
Deals with frontend ajax requests
Notice: 
After release of WordPress 4.7 this plugin is a bit obsolete.
In WordPress 4.7 users are able to select their preferred locale (language) when editing their profile. Plugin add exact same functionality to pre 4.7 WordPress.
I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale(). 
<a href="https://make.wordpress.org/core/2016/11/07/user-admin-languages-and-locale-switching-in-4-7/" rel="nofollow ugc">More info</a>

Admin Language Per User

a must have for multilingual sites

Switch your backend administration panel in any installed language per user basis.

Akshar Soft Solutions

This plugin lets users change their administration language to native English (en_US locale).
This is useful during site development and for people more accustomed to the english administration panel, even if your site 
is in another language. (The frontend will still use the native language.)
This plugin is fully compatible with WooCommerce and can correctly identify and translate frontend / backend AJAX requests. 
If you are using this plugin with Advanced Custom Fields, please move the as-english-wp-admin.php file to the /wp-content/mu-plugins/ folder 
(create it if it does not exist). This is an ACF limitation.
This plugin is developer friendly and small (~200 lines of code). Check the FAQ for customization examples.
Usage
Basic usage
Once you have installed and activated the plugin, navigate to any admin page and check the top admin bar. A button will 
display your current locale. If you click on it, the admin will change to English locale (en_US). To switch back, 
press the button again.
<h3>Requirements</h3>
<ul>
<li>PHP 5.3 or higher</li>
</ul>
<h3>Translations</h3>
<ul>
<li>None</li>
</ul>

AS English Admin

Lets users change their administration language to English

Admin Language Per User vs AS English Admin