I’m ashamed to say that I don’t know much about translating apps, even after writing an app with over 30 translations. When you’re part of the gnome project, they just appear by magic and I have had my head somewhat in the sand since when trying to test I invariably got:
% LANG=otherlang.utf8 ./myprog (process:20007): Gtk-WARNING **: Locale not supported by C library. Using the fallback 'C' locale.
A little googling led to various command line invocations of locale-gen & suggestions of hand editing config fies by people who didn’t understand it all either.
So I’m glad to say that all thats needed (in ubuntu 9.04 anyway) is really to check a few boxes in
System->Administration->Language Preferences->Install/Remove Languages