About [[MediaWiki:Citethispage-content/en]]

CLOSED This issue is closed by Nemo_bis

The documentation reads "Any wikilinks in this message point to pages on the local wiki, so they must be translated."

Does this mean that this page ignores the chosen interface language? Otherwise using an interface language which differs from the site language would automatically break all the links.

Yes, it's correctly used in content language:

SpecialCiteThisPage.php
124:            $msg = wfMessage( 'citethispage-content' )->inContentLanguage()->plain();

I assume this further then means that none of the translations are useable outside of Wikipedia-wikis? (unless they create the same pages)