Before getting the top level parent page, you could check if the page has parents; also you forget to define $template
.
This code should work:
function load_usetemplate( $template ){
global $post;
if( $post->post_parent > 0 ) {
// get top level parent page
$parent = get_post( reset( array_reverse( get_post_ancestors( $post->ID ) ) ) );
if( $parent->ID === pll__(8) ){
if ( $child_template = locate_template( 'page-joinery-subpages.php') )
return $child_template;
}
}
return $template;
}
add_filter( 'page_template', 'load_usetemplate' );