This ancient MediaWiki extension provides two magic words:
{{pagelang}}returning the page content language{{intlang}}returning the interface language
As of MediaWiki 1.29, the functionality of {{pagelang}} is provided as part of MediaWiki core in the form of {{PAGELANGUAGE}}, which makes this separate extension obsolete.
(Also, as of MediaWiki 1.35, ParserGetVariableValueSwitch hooks must manually cache their returned value. This is incredibly silly, and provided yet another good reason to just let this extension die.)