yesno: Always accept English even when localised.
It seems to be a failing of i18n in unix that the translation stops at the
commands and the parameters to them, and ikiwiki is no exception with its
currently untranslated directives. So the little bit that's translated sticks
out like a sore thumb. It also breaks building of wikis if a different locale
happens to be set.
I suppose the best thing to do is either give up on the localisation of this
part completly, or make it recognise English in addition to the locale. I've
tenatively chosen the latter.
(Also accept 1 and 0 as input.)