- π¦πΊAustralia pameeela
This can't be reproduced currently in vanilla Drupal. If you enable language detection, and try to visit a node alias with a lang path that doesn't have a valid translation, you get a 404 because the alias doesn't exist in the given language. That's also why the 'computed path' (e.g. /{lang}/node/{nid}) works -- because this is a valid path.
Perhaps this has changed in the meantime but I suspect more likely this was something custom/specific to this website, since there are no other reports of it.