Currently default text of «MediaWiki:Citethispage-content» is called "MediaWiki:Sitesubtitle" (in form {{int:sitesubtitle}}) - see as example https://inh.wikipedia.org/w/index.php?title=MediaWiki:Citethispage-content/en&action=edit
This is problem, becouse message "MediaWiki:Sitesubtitle" early was created by default, but from 2007 this is deleted as "No longer required" - see https://ba.wikipedia.org/wiki/MediaWiki:Sitesubtitle
In the enwiki, dewiki etc this message was created locally, especially for using in the «MediaWiki:Citethispage-content» (and therefore in the «Special:Cite»).