As reported by abias, when deleting an activity with JavaScript turned off, the string deletecheckfull | core is displayed:

'Are you absolutely sure you want to completely delete the user {$a}, including their enrolments, activity and other user data?'

It should use the same string as when JavaScript is turned on i.e. deletechecktypename | core:

'Are you sure that you want to delete the {$a->type} "{$a->name}"?'