A.D.

Sometimes we want the front end of our blog to be translated for the users, but keep the backend admin panel in English. Admin in English with Switch does just that. It lets you switch your backend administration panel to English from admin toolbar, even if the rest of your blog is translated into another language. This plugin allows turning English language on and off for each user’s back-end independently.
For more information about original Admin in English plugin please visit https://wordpress.org/extend/plugins/admin-in-english/ page.

Admin in English with Switch

This plugin is an extended version of Admin in English plugin (v1.2) developed by Nikolay Bachiyski. See the Description for more details.

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I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale().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.

Admin in English with Switch vs Admin Language Per User