CKEditor Language-plugin

The language plugin doesn't load when added in Hippo 10 (or 7.9).

I've downloaded the Language plugin (version 4.4.8) and added it in the cms according to the documentation but it doesn't load. When I add the same plugin to the plain CKEditor (v 4.4.6) it works fine.
So it has to be something in the customization for Hippo that breaks the plugin.

Other plugins that I've added in the same way work fine.