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I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale().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

NowSquad

This widget loads an www.englishleveltest.com widget inside an iframe with the theme as a parameter, bright and dark posibilities. Unlike the full www.englishleveltest.com version this one does NOT require any contact information to your visitors. Also a “powered by” link is displayed below the leveltest, it can be disabled from the widget configuration.

English Level Test

Offer your blogs visitors the posibility to take an english level test according to the…

Admin Language Per User vs English Level Test