We now have a new Web localization dashboard for Mozilla :
On this page, localizers can see what is left to do in terms of web localization on mozilla.com/europe and subscribe to the RSS feed for their locale.
You can also look at completion per project on specific bugs (like "55% of locales have translated the page").
There is more information in our mozilla.dev.l10n.web newsgroup and you can discuss the tool there.
I have tried to make a very simple tool to use and maintain, it's not perfect, it does not track everything yet, but the early feedback is positive (and it will definitely improve and become more comprehensive over time).