MediaWiki:Apihelp-query+allmessages-param-includelocal
From Global Knowledge Compendium of Traditional Crafts and Artisanal Techniques
Also include local messages, i.e. messages that don't exist in the software but do exist as in the MediaWiki namespace. This lists all MediaWiki-namespace pages, so it will also list those that aren't really messages such as Common.js.
