Manual:$wgExtraInterlanguageLinkPrefixes

Details
By default, only treats interwiki links as interlanguage links (i.e. moved to the "Languages" section of the sidebar) if the interwiki prefix matches one of the ISO language codes known to MediaWiki. From time to time, you may need to set up extra prefixes that are to be treated as interlanguage links (e.g. "mul" for a multilingual site).

This setting is ignored if is not set to true.