Yes, the site has not been updated for a while and thanks to @Luke, it’s building again. But we have a deployment problem on GitLab pages and we don’t know where it’s coming from so right now, I’m updating the sites “by hand” (with rsync) when it’s needed. So we should fix the links bar but deploy manually.
I did it a while ago (in may 2018) but it was not merged. Since a lot of things happened inbetween, I decided to refound the existing content into a new site (duniter.fr) with the goal of organizing it better. Today, I do not think that i18n-ing a website is a good strategy since it gives less freedom on editorial choices and they have to move a lot. That’s why I think it’s better to translate the website et reorganize it while doing so. The improvement can still be shared and discussed between different websites maintainers.
You’re right, it links to https://duniter.org/fr and should link to https://duniter.org/fr/ which redirects to https://duniter.fr/. You can fix it, thanks 
(but I’ll have to deploy by hand until I fix the GitLab pages bug).