It doesn't have to be enforced algorIthmically. It can just use a client-side spellchecking library that makes spelling errors very visible. When it detects > $THRESHOLD errors, then it shows a warning saying, "Your post will be partially hidden if you don't fix your spelling/grammar errors." Then, on the client-side, it changes the opacity to something like 0.5 for posts with > $THRESHOLD spelling/grammar errors.