Sam Bull

Sets the site locale dynamically according to visitor preference. 
This preference comes from the user’s browser.
To install all available translations click the ‘Install All’ button in Settings->General. 
To install individual locales, select the site language and save changes for each locale you want to install.
Technical details: 
 Will parse the Accept-Language HTTP header and find the best matching language for the user from the translations available on the site.

Preferred Locale

Select site locale/language according to user preference (set in their browser).

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.

Preferred Locale vs LanguageBack