I have a situation where I need to modify
   an existing program so that the language
   presented in the GUI is in Japanese instead
   of English.

I suggest that you take a look at the GNU gettext library for
internationalization of C programs.  The latest released version is at
ftp://ftp.gnu.org, and alpha/beta releases of yet-to-be-released
versions are at ftp://alpha.gnu.org.