This is not really easily possible. What you can do is look at / copy the source code of any standard launch page (open the page in your browser, right-click somewhere on the page and select "View Page Source"). The code for the installer panel is in there, you would copy it, translate it and then incorporate it into your own, custom HTML launch page. The easier and more robust option would be to just include French instructions for downloading in the default launch page's body _in addition_ to the installer download panel (in English).
 That option will be named slightly differently depending on which browser(s) you use.