OK, thanks. I came up with the following that seems to work to resort them.

add_filter( 'editable_roles', 't5_sort_editable_roles' );
/**
 * Array of roles.
 * @wp-hook editable_roles
 * @param   array $roles
 * @return  array
 */
function t5_sort_editable_roles( $roles )
{
	uasort( $roles, 't5_uasort_editable_roles' );
	return $roles;
}
/**
 * Compare translated role names.
 * @param  array $a First role
 * @param  array $b Second role
 * @return number
 */
function t5_uasort_editable_roles( $b, $a )
{
	return strcasecmp(
		translate_user_role( $a['name'] ),
		translate_user_role( $b['name'] )
	);
}