Wikipedia articles are different in every language, and not all of them appear in the English version of Wikipedia. Firefox 3 includes in many languages versions (Spanish, German, etc., but not French) a search plug-in for Wikipedia set for the corresponding language. Download the version of Firefox 3 for the language you're interested in using (no need to install), mount the disk image, control-click on the Firefox icon and choose Show Package Contents from the pop-up menu. Navigate to Contents » Resources » MacOS » searchplugins; the Wikipedia plug-in is an XML file that you can copy and install into your main language's version of Firefox. Just copy and paste the file from the language of choice to the same folder in your main copy of Firefox.
If, for the language you are interested in, Firefox does not include a Wikipedia plug-in, you must modify one. Quit Firefox, then duplicate the English plugin (wikipedia.xml). Next, change the name of the duplicated file to wikipedia-yl.xml, where yl should be the language for the search engine (fr for French, for instance). Open the file in a text editor (TextEdit, BBEdit), and change all en instances with the language code you are interested in, including the en in the ShortName entry. Save and launch Firefox.
This technique may also be used with other search engines, such as Yahoo and Google, that offer country-specific search engines.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080619084039715