i18n: Correctly resolve en_GB if not explicit
i18n: Correctly resolve en_GB if not explicit

negotiate_locale(...) is pretty picky and en_GB won't be returned even
if en or en_US are provided as preferences. fr_FR or es_ES end up being
returned instead.

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