#25 Begin: Fix language contexts

[Foster Brereton]

Language contexts in Begin are implemented such that
the current language is propagated to every string that
is localized thereafter. Language contexts should be on
a per-window basis, not a global setting within the
IDE. What can I fix without having to haul around the
language context? A value per-window may not be
necessary (e.g., cache button label names instead of
going out to xstrings every time)