Manual:$wgNamespacesWithSubpages

Details
The variable holds an array of namespaces, numbered from -1 (Special:) and not including -2 (direct-linked media) indicating which namespaces allow sub-pages. The array is indexed by the numeric namespace ID, and each index holds either 0 (no sub-pages) or 1 (sub-pages allowed).

See Help:Subpages

Default value
Note: Each version of MediaWiki only went as far as the maximum namespace existing at the time.

Prior to 1.5.0
The NS_ constants weren't used (possibly because they weren't defined) so numeric values were used instead:

Enabling for a namespace
An easy way to enable subpages for a given namespace is to edit the  and insert the following:

Namespace names
The names for namespaces are set in files such as those listed in Locales for the Wikimedia projects, with default Language.php: