I'm afraid CKEditor is basically unsupported.
You can try setting up VisualEditor
<https://www.mediawiki.org/wiki/Extension:VisualEditor>, which is the
latest in WYSIWYG editors for MediaWiki. It depends on a Node.js service
called Parsoid, though, so you have to be able to setup it on your
server to use VisualEditor.
--
Bartosz Dziewoński