This message is currently identical to MediaWiki:Pagecheckout-ui-workflows-inspector-activity-page-checkout-property-user-help/en: "For whom page will be checked out". The context suggests that "checked out" should be "checked in" in this message.
Also, both messages need a definite article ("the page").
The extension that this applies to is "Page Checkout". There does not seem to be a tag for it.