Manual:$wgNamespaceAliases

Details
These are alternate names for the primary localised namespace names, which are defined by $wgExtraNamespaces and the language file. If a page is requested with such a prefix, the request will be redirected to the primary name.

Set this to a map from namespace names to IDs. Example:

$wgNamespaceAliases = array(       'Wikipedian' => NS_USER,        'Help' => 100,    );