Update or drop the show manual logic in torbutton
toolkit/torproject/torbutton/modules/utils.js has a function to enable/disable the manual link, depending on user's language:
let availableLocales = ["de", "en", "es", "fr", "nl", "pt", "tr", "vi", "zh"];
The list seems quite old, and I wonder if linking to the bundled manual even when the translation isn't available is better than not linking to any manual at all (I'd say yes).
Otherwise, we should update that list (and it would be yet another list to keep in sync, unless we add some code on the manual patch, but I'm not sure is worth it).