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I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale().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

mendezki

Note: This plugin was intended as a quick fix for displaying various english dates in german. It unfortunately does not fully cover all date displays.
Using an english WordPress install and publishing posts in german? Those pesky English dates produced by <code><?php the_time(); ?></code> were getting on my nerves too, hence this plugin. Automagically converts english months and days (except for the initials, so far) to their german equivalent.
Intended usage:
Replacing dates like ‘Thursday, October 22nd’, for instance in your custom built theme or themes that just use the date without ‘posted by..’. Please note: It doesn’t completely ‘deutschify’ your theme’s date representation.
Example: Where some themes would say ‘Posted by Annemarie on Thursday, October 22nd’ only the date part is translated, resulting in ‘Posted by Annemarie on Donnerstag, Oktober 22.’. For complete deutschification one would either, depending on your specific situation:
<ul>
<li>Edit Date Format settings under the Settings > General tab in your WordPress admin area.</li>
<li>Edit your theme’s template files, adapting the <code><?php the_time(); ?></code> template tag appropriately. For a howto on editing the time template tag, check the <a href="https://codex.wordpress.org/Template_Tags/the_time" rel="nofollow ugc">Template Tags/the time Codex page</a>.</li>
<li>Or, as an alternative to this plugin, use a german .po language file.</li>
</ul>
Dutchify in stead of Deutschify?
For the Dutch version of this plugin, visit the <a href="https://wordpress.org/extend/plugins/dutchdate/" rel="ugc">Dutchdate Plugin Page</a>

DeutschDate

Replaces english day and month names with their german equivalent

Admin Language Per User vs DeutschDate