If you want the default website to be French, yea this is possible, and the English version will be /en or whatever URL you prefer.
No, the multisite is not working right now, the plugin is only activated, but i am not able to create a new working one.
If you take a look here: http://www.jamhourendowment.org/wp-admin/network/sites.php
You can see that I have created a new site but when i try to visit it, it’s 404. And that’s the issue, the new website is detected as a broken(not found) page.