Manual:$wgMetaNamespace/ja

詳細
The name of the meta namespace (also known as the project namespace), used for pages regarding the wiki itself. If set to false, the value of  is assumed. An alias for this namespace is "Project". The associated talk namespace is specified by .

Be sure to use underscores instead of spaces, otherwise you will have problems. It is however ok to use spaces in.

例
If you wanted to use a meta namespace of "Project", you would put in :