This bug causes citoid to "disappear" from a wiki and for the default VisualEditor citation tools to appear instead.
For some reason, the message goes randomly missing for some languages; if this happens to be the site language, then this causes citoid to be missing for most users.
The site language has gone missing in the past for both en wiki and sv wiki.
This can be fixed for any given wiki by clearing the cached version of the message. Most logged-in users have the correct permissions to clear the cache. You can do this using the api by doing:
(Substitute your site's language prefix for 'en' here.) Please continue to report outages here as well even if you fix them, as we don't know what causes the issue and reporting outages can help with diagnostics.