How to: Multilanguage site, but single language backend?
I know this post is getting old and the OP probably has this sorted by now but I came here looking for the same answer and thought I'd post a follow up for anyone looking for details. I've made this as easy and basic as possible in case you're new to Drupal.

My version is 7.21.

Ok, here's what I just did to get the back end in English and the site content in Spanish. I'm assuming you have all the translating modules installed and set up, etc.

Go to your admin profile and set your languages, for me its this:

Administration Language = Default administration language, currently English

Language Settings
Language = English

Then go get the Administration Language module that Patrick linked to. (its currently the Dev version but works fine for me, look in the red area for the DL). Install and enable.

Then go to Administration > Configuration > Regional and language > Languages:

Scroll to the bottom and you'll have an option of "Use administration language in the administration menu". 
Set that to Enabled

Save it and now everything should show in what ever language while the back end stays in the language you set for your admin profile.

I hope this helps!