I am a freelancer and build sites powered by WordPress, bbPress & BuddyPress.
Recently I took work from a client who operates a WordPress installation in a language other than English and it was a productivity killer for me as I wasn’t able to browse it quickly.
WordPress Dashboard in English language
For me Admin in English plugin does the job after installing and activating it, the WordPress dashboard was back to English language keeping the original language intact on the rest of the WordPress install i.e. the front end of the website / blog (what user sees).
WordPress Dashboard in different language
In case you want more control and also wish to provide user the preference to chose their preferred language, then you can try WP Native Dashboard plugin.
This plugin enables you to give users the option to select a preferred language. Pretty cool!