(Follow up from plwiki's technical village pump "pl-PL" )
A template generated in the automatic mode for page:
receives lang = pl-pl language code.
We have modified our {{cite web}} template (https://pl.wikipedia.org/w/index.php?title=Modu%C5%82%3ALang&type=revision&diff=44015091&oldid=40741992) to accept this, but it looks like an incorrect language code.
The target page contains <html lang="pl" ... opening tag.