Manual:$wgNamespaceAliases/ja

Details
These are alternate names for the primary localized namespace names, which are defined by  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:

To add a single alias:

If you need an alias for the project you probably need a similar alias for its talk page and you should add an array as above or two lines:

Prior to MediaWiki 1.17, it was impossible to make an alias to NS_MAIN.

Do not use spaces
Use underscores instead of spaces in namespace names. 'My Namespace' is an invalid name; use 'My_Namespace' instead.