A.D.

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 with Switch does just that. It lets you switch your backend administration panel to English from admin toolbar, even if the rest of your blog is translated into another language. This plugin allows turning English language on and off for each user’s back-end independently.
For more information about original Admin in English plugin please visit https://wordpress.org/extend/plugins/admin-in-english/ page.

Admin in English with Switch

This plugin is an extended version of Admin in English plugin (v1.2) developed by Nikolay Bachiyski. See the Description for more details.

Sergey Biryukov

Creates a separate tiny .mo file to use on a site front-end. Allows to save some amount of RAM on a shared hosting server.
Thanks to MAX for the original non-plugin solution and to AlexPTS for the idea.

WPLANG Lite

Creates a separate tiny .mo file to use on a site front-end.

Admin in English with Switch vs WPLANG Lite