Wordpress - How to set back-end language per user?

Have you seen http://wordpress.org/extend/plugins/wp-native-dashboard/?

I think it does exactly what you are asking.

Also 'interesting' is http://wordpress.org/extend/plugins/in-their-language/ which attempts to set the locale automatically based n browser settings.

Starting with Wordpress 4.1 (maybe even earlier, I don't know) this functionality is built-in (no need for a plugin). Simply:

1. Download the language pack of your choice from http://wpcentral.io/internationalization/
2. extract the content into wp-content/languages (create the languages folder if it does not exist yet)
3. Done!

Logon as the editor who needs a different language, and go to his/her profile and set the Admin Language.