Bug 6629 : Sanitizing input from language cookie

Bug 6629 : Sanitizing input from language cookie

I dont think we can use only 2 digits, some languages is much longer
zh-hans-TW for example

But the regex should stop it bening able handle nasty chars,
whitelisting safe ones instead

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
I checked the patch doesn't break language switching and language selection.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
I confirm the bug security issue was not here for master, but this fix improve the behaviour, so pushing it