This is how you can get the post object of a page with a matching path, minus the /en part:
$page_path = get_page_uri( get_queried_object_id() ); // en/page/child-page
$target_page_path = str_replace( 'en/', '', $page_path ); // page/child-page
$target_page = get_page_by_path( $target_page_path ); // WP_Post of target page.