水野史土

This plugin enables to see the source and the target of each translation word/phrase/sentence. When you use other than English locale, all messages that are translated via gettext will be shown in both the source language and the target language. For example, “error” will be shown as “error” + “erruer”
Since version 2.0, you can display glotpress links. Go to Settings -> Reading, and you will find ” Show GlotPress link” checkbox.
<a href="http://ounziw.com/2011/06/28/english-japanese/" rel="nofollow ugc">Japanese description</a> is also available.
CAUTION: This plugin is used for debug. DO NOT ACTIVATE ON A LIVE SITE!

Debugging Translation

Messages are displayed in both the source language and the target language.

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I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale().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

Debugging Translation vs Admin Language Per User