This one is really a cool waste of time for the bored/geeked/pedantic ... or whatever stereotype you choose use. But it may come in handy, especially if one suddenly finds themselves emigrating to a new language, and you do a lot of Mac troubleshooting and would like to improve your language skills at the same time.
For each application, open its Get Info window (Command-I with the app selected in the Finder). In the Languages section, turn off all languages other than the one you're interested in using. So for French, turn off all the languages except for French. Launch the app, then take some screenshots of the menus and other UI elements that are now in another language. When done, revisit the Get Info window and return your app's Language section to your native tongue.
Save all these shots to a folder. Then when you are bored, feeling a bit geeky, or whatever, use Quick Look (or Preview, etc.) on your shots and see how they compare to your regular menus and UI elements.
Just finishing an audio CD course by Michel Thomas (and I still have a lot to learn), he suggests that in reading, one should try to read what is of interest to the student. So if you are into Macs, I guess this would be good stuff to read!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090416004519162