I found a link I think we all should keep in mind:
Documentation Status - https://developer.mozilla.org/en-US/docs/MDN/Doc_status/B2G_OS
Under the localization section, it’s possible to find more information on single language translation status (regarding untraslated, outdated and updated pages).
Take a look at the French page to have an example.
Maybe we should pin this post, adding other links of general interest.