Even if you buy a Mac in France, English and Japanese are the only languages available in Dictionary.app. MacOS X is the same no matter where it is sold and only comes with English and Japanese pre-installed at the present time.
There is a translator Dashboard widget that comes with Leopard that can do French-English and vise versa, as well as a number of other languages, but it would be nice to have the capability in the Dictionary as well.
There is a freeware Dictionary development kit available from Apple that is included with the Developer Tools and is located in /Developer/Extras/Dictionary Development Kit. [crarko adds: Documentation is available here.]
Some language modules are also available from third parties. You can install the good 'Dictionnaire-AcademieFrancaise'
by following this link.
[crarko adds: I tested this, and it works as described. Unzipping the downloaded file gives an Installer package that installs and loads the French dictionary into Dictionary.app. There is also an earlier hint about enabling the built-in Japanese dictionaries, which describes how to load/unload the language files.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100809085441916