Separate entity and interface language code

The entity language code is the one you play the game in; the interface
language code is the one we show the interface in. Right now, all the
messages are hard-coded in English, so while it’s nice to have the code
use Intl for the formatting already, it doesn’t really make sense to
show localized numbers in between English text (“You successfully
identified Q64 in ১ guess with an accuracy of ১০০% and a time of
০:০০:০১.”), and it seems more sensible to always pass 'en' into the Intl
constructors. But this way, once the interface language can be
customized, some of the rest of the code will be ready for it.