Manual:$wgExtraGenderNamespaces

Details
Same as $wgExtraNamespaces, but for namespaces with gender distinction (user space). You have to define this by yourself if you have extra namespaces which need gender distinction; standard MediaWiki namespaces' gender variants are defined via  in each language's MessagesXx.php.

the default form for the namespace should also be set using $wgExtraNamespaces for the same index.

Example: In this example the namespace name fallbacks to the MediaWiki default, so nothing needs to be added to.