From the pages of 'crazy things you can do with Drupal 8' comes this idea that I recently had to implement on a site. This was on a site that had implemented translations, but they needed something a little extra. The central idea was that if you visited a page on the site in a particular language, and that language didn't exist, then to try and give the user a similar language.
If you weren't already aware, Drupal's default behaviour is to return the default language version of a page if the language requested doesn't exist. If the Path Auto module is installed (which is usually the case on most Drupal sites) then Drupal will not be able to translate the path alias correctly and will issue a 404 if the requested translation doesn't exist. Drupal can't take the path for the French page /fr/about-us and find the original node because as far as it's concerned /fr/about-us doesn't exist.