Sounds like it could be a possibility.

Where does this stand currently?

The biggest difficulty in this idea is that users are able to set their language in MediaWiki, but nothing about the HTTP environment carries forward that language setting, so when an HTTP request is made and hits an error page, there's little-to-nothing to go off of in that request to indicate what the target language should be as it isn't querying their MediaWiki user preferences. When I think of ideas for creating a localized error page in a wiki environment, Wikipedia comes to mind with their localization in the subdomain (e.g. en.wikipedia.org, fr.wikipedia.org, etc), where you could theoretically make an nginx if statement that picks up the subdomain and returns a different error_page directive depending on the subdomain. However on ShoutWiki where the domains don't indicate the site language, that doesn't work. Even if you were to query the site's content-language response header before returning a PHP-based error_page, the site's content-language could be different than the user's customized preferences.

Last reported 7 November 2017 here by 3 users.