Change Dropdown Language to Flag display

Imported From: http://groups.google.com/group/in-portal-dev/browse_thread/thread/2b7eca307b4dc25d#

I would like to use the actual lang flag displaying system (like we can see in multilingual fields)  to replace the dropdown lang selector on front, in header.

Would you please help me, I'm sure it's not a big deal, but I didn't succeeded by myself to do it.