The RelatedSites extension is used in Wikivoyage to put links to sister projects and other stuff in the sidebar.
It uses $wgExtraLanguageNames to allow links like [[wikipedia:Italy]] to be put into the ParserOutput language links array. Later in ParserBeforeTidy, these array elements get unset and these "language" links get passed to output page.
Somewhere in the process, these get passed to LangLinkHandler in Wikibase and causes a php notice:
Notice: Failed to map interlanguage prefix wikipedia to a global site ID. [Called from Wikibase\LangLinkHandler::localLinksToArray in /srv/mediawiki/extensions/Wikidata/Wikibase/client/includes/LangLinkHandler.php at line 337]
This apparently happens on quite often on Wikivoyage but php notices do not get logged so no one has noticed.
Perhaps best way to solve this is to improve how RelatedSites work, using something other than the language links array. RelatedSites could also take advantage of Wikibase / Wikidata to provide these links.