Manual:$wgInterwikiMagic

Details
Treat language links as magic connectors (interlanguage links), not inline links. If $wgInterwikiMagic is set to true and $wgHideInterlanguageLinks is set to false, interlanguage links (e.g. fr:La Foo) will show up in the sidebar.

An interwiki link is considered a language link if the interwiki prefix matches a MediaWiki language code.