ProjectBuilder can do this, actually. What you do is make multiple .nib and .strings files for the languages you want to use. They all get built into the app, which will then choose the language that's most appropriate, based on the language the user's OS is set to. It'll fall back to English if the appropriate language isn't provided.
Sadly, although OSX apps are very easy to localize, actually doing the localization isn't something many programmers will undertake. There are some legitimate reasons for this: a programmer can't localize a program into any languages he doesn't know (at least, not without outside assistance).
If you're trying to localize an app you didn't write, things are a bit trickier. You'll have to copy the .nib and .strings files, edit them to your liking, and package them into a "Language.lproj" folder. Then insert this into the app's package (you don't have to remove the original), and you should be set to go.
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