Simplify session.lang getter
Simplify session.lang getter

This also fixes the error in #3001 if - for some odd reason - a user has
a None language in their settings.

ThiefMaster/indico,DirkHoffmann/indico,mic4ael/indico,mic4ael/indico,ThiefMaster/indico,ThiefMaster/indico,indico/indico,mvidalgarcia/indico,mic4ael/indico,mvidalgarcia/indico,OmeGak/indico,indico/indico,OmeGak/indico,DirkHoffmann/indico,mvidalgarcia/indico,pferreir/indico,mvidalgarcia/indico,pferreir/indico,pferreir/indico,mic4ael/indico,DirkHoffmann/indico,indico/indico,OmeGak/indico,ThiefMaster/indico,pferreir/indico,indico/indico,OmeGak/indico,DirkHoffmann/indico