Auto Create Category Pages has the following 3 messages in the Mediawiki
namespace, which can be changed to accommodate your preferences:
{|
| autocreatecategorypages-stub
| Used as stub when creating a category page. Parameters: $1 - page title
|-
| autocreatecategorypages-createdby
| Used as an "edit summary" when creating a category page.
|-
| autocreatecategorypages-editor
| Used as username when creating a category page automatically.
If the user does not exist, it will be created automatically.
|-
|}

The text of 'autocreatecategorypages-stub' can also be overridden using
$wgAutoCreateCategoryStub. This can contain wikitext as well.