unset LANGUAGE for showing [Y/n] answer hints

unset LANGUAGE for showing [Y/n] answer hints

nl_langinfo is used to acquire the YESEXPR of the language used,
but it will return the one from LC_MESSAGES, which might be different
from the language chosen for display of the question (based on LANGUAGE)
so this commit removes the [Y/n] help text from the questions itself and
moves it to the prompt creation in which the usage of LC_MESSAGES is
forced for it, so that the helptext shown actually represents the
characters accepted as input for the question.

There is still room for problems of course starting with an untranslated
"[Y/n]" but a translated YESEXPR or the problem that the question is
asked in a completely different language which might have a conflicting
definition of [Y/n] input or the user simple ignores the helptext and
assumes that an answer matching the question language is accepted, but
the mayority of users will never have this problem to begin with, so we
should be fine (or at least a bit finer than before).

Closes nothing really, but should at least help a bit with bugs like
deb:194614, deb:471102, lp:1205578, and countless others.