Louy Alakkad

This plugin gives you the ability to run the site in a language (Arabic for example) and use the admin panel in another (like English).
Just go to Settings » General and choose your language.

Admin Locale

This plugin allows you to change the language of the admin panel without changing the whole site language.

moxu

I created this plugin because i wanted this feature (to set a specific locale for frontend) out of my themes. I’m assuming i’m not the only one who would preffer to have the backend en_US only, so here it is…
It’s not tested with other versions of wordpress except 3.4, but it should work without a problem at least from 3.0 onwards.
This plugin assumes you’re not using a translation plugin , so, if you have bilingual or multilimgual blogs, use with caution (and let me know … especially with qtranslate 🙂 ).
I’t not multisite tested/ready yet.

LanguageBack

Simple , minimalistic plugin that lets you set different locale for frontend and backend.

Admin Locale vs LanguageBack