If you look inside the contents of an application bundle (right click: Show Package Contents), in the Resources folder, you'll see folders named "en.lproj" (English); "es.lproj" (Spanish) and so on.
Each one contains the interface files (nib files) for that language. So that's the menus, dialogs, tooltips, windows, palettes, etc.
You need to use Xcode to create these files. You used to be able to open nib files in apps in Xcode, but they are now compiled so you need the source.
In short: it's not as easy as modifying some text files.