Aaron Edwards

Tired of your custom plugin and theme translation files getting wiped out every time you use the WordPress auto-updater? Install and activate this plugin and 
WordPress will look for missing translation files in the WP_LANG_DIR where you keep your WordPress translation files.
<h4>UPDATE</h4>
In WordPress 3.8 and greater this plugin’s functionality is included in core. You can put translation files in: 
* Plugins: /wp-content/languages/plugins/textdomain-locale.mo 
* Themes: /wp-content/languages/themes/textdomain-locale.mo
A free plugin by Aaron Edwards of <a href="http://uglyrobot.com/" rel="nofollow ugc">UglyRobot Web Development</a>.

Translation Preserver

A simple plugin that helps preserve your custom plugin translations through auto-updates by allowing you to put them in your WP languages folder.

Nikolay Bachiyski

Sometimes we want the front end of our blog to be translated for the users, but keep the backend admin panel in English. Admin in English does just that.

Admin in English

Admin in English lets you have your administration panel in English, even if the rest of your blog is translated into another language.

Translation Preserver vs Admin in English