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I still maintain it for people with outdated WordPress sites. And the plugin is still usefull for sites with outdated plugins that not obey get_user_locale().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

Bernhard Kau

Backend Localization is deprecated and will be removed from the plugin directory on September 2, 2023, 14 years after its first release. With WordPress version 4.7, released in December 2016, <a href="https://make.wordpress.org/core/2016/11/07/user-admin-languages-and-locale-switching-in-4-7/#content" rel="nofollow ugc">the main purpose of this plugin</a> was integrated into core. With WordPress version 5.9, released in February 2022, <a href="https://make.wordpress.org/core/2021/12/20/introducing-new-language-switcher-on-the-login-screen-in-wp-5-9/#content" rel="nofollow ugc">the only useful missing feature</a> was also integrated into core.
Thanks to everyone who used the plugin, gave constructive feedback, rated it and send me messages on how it helped them with their sites. This was my second WordPress plugin, and it’s surprising still has 2000+ active installations. If you see this message, you are one of them. But now feel free to remove it from your site. You can find my other plugins <a href="https://profiles.wordpress.org/kau-boy/#content-plugins" rel="nofollow ugc">on my WordPress profile page</a>.
This plugin enables you to run your blog in a different language than the backend of your blog. So you can serve your blog using e.g. German as the default language for the users, but keep English as the language for the administration. 
You can choose the language you want to use from all installed language files or you can install additional languages.
A list of all of my plugins can be found on the <a href="http://kau-boys.com/wordpress-plugins" title="WordPress Plugins" rel="nofollow ugc">WordPress Plugin page</a> on my blog <a href="http://kau-boys.com" rel="nofollow ugc">kau-boys.com</a>.

Backend Localization

Fantastic plugin if you need to deal with websites in various languages, but want to keep the backend in your preferred language. I can highly recommend this plugin.



Works great

Although WordPress 4.7 introduces the user language feature, this plugin is still tremendously helpful in my customer support job when I have to log in on sites running WordPress in an older version than 4.7. 

One feature that the WP 4.7 core user language doesn’t provide is to pick your language upon login—which makes this plugin even more helpful in customer support, since navigating for example an RTL back-end to find the profile page can be quite time-consuming when you're not used to it. (I assume the same is true for LTR for other people.)

All in all, this plugin is a huge win. I wouldn’t want to miss it in my daily job!

Still tremendously helpful

This plugin enables you to run your blog in a different language than the backend of your blog. So you can serve your blog using e.g.

Admin Language Per User vs Backend Localization