Luckily, X-Mouse Button Control supports the French language, but you'll have to install it. The language is available in form of a module. Access the following page then click on the French link on the left side to download the French.xmbclp file. If you have X-Mouse Button Control installed, double click the file and it will install automatically.
Restart X-Mouse Button Control and then you have the possibility to choose the French language from its Settings menu.