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I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale().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

martinjuhasz

Lets you choose your Backend Language for your complete Backend, also if your Frontend is using another Language (including Multisite). 
You can choose from any language you installed. For a list of availible languages, check <a href="https://codex.wordpress.org/WordPress_in_Your_Language" rel="nofollow ugc">“WordPress_in_Your_Language”</a> or create your own files.

One Backend Language

Lets you choose your Backend Language for your complete Backend, also if your Frontend is using another Lang (including Multisite)

Admin Language Per User vs One Backend Language