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,    );

Do not use space in namespace names
Take care: When you define a namespace or set a name of a namespace to somthing which contains space character you should use underline inspite of it. For example 'My Namespace' is an invalid name and it might provide you many problems and you should use 'My_Namespace' instead of it.