EZP-23546: adding language switcher to DemoBundle

Language switcher was missing in the demoBundle.
I just added it into DemoBundle for eng <-> fre siteaccess as an example.
There is no design (for bootstrap3, please see http://share.ez.no/blogs/huck-florent/language-switcher-using-bootstrap3/(language)/eng-GB). 
It only display a list with flags and siteaccesses name on the top right corner.


(@lolautruche correct me if i'm wrong) As we use SF routing to get path to siteaccess, we need to convert _locale from SF (en) to eZ language (eng-GB).