[Debian] Required steps to get localized GUI


I’ve got an Asterisk 1.8/Freepbx 2.10 setup.
I can see a scrolling list (in the top right corner of every Freepbx screen) holding various language options but none seems effective: everything remains in english.

I can see various amp.po files.

What are the required steps to get a functional and localized GUI ?

For those reading french, I basically followed the steps mentioned in http://www.queret.net/blog/2011/09/asterisk-freepbx-installation/