When using Special:Import and then import an unknown content model, the exception MWUnknownContentModelException is thrown and import stopped, but the text is not localized to the user language when showing on Special:Import
On de I get (message importfailed):
Import fehlgeschlagen: The content model 'sanitized-css' is not registered on this wiki. See https://www.mediawiki.org/wiki/Content_handlers to find out which extensions handle this content model.
Please add a message for it to show a localized error text to the user