404 Errors keep increasing

Migrated to WP several months ago. 404 errors just keep on increasing (now at 34,000). Redirected via htaccess but 404 errors remain.

RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(es|en|de)/.+-([0-9]+)\.html$ ?antiguoID=$2&antiguoLANG=$1 [L]

en php (/wp-content/themes/citygonow/header.php):
if(isset($_GET[‘antiguoID’]) && isset($_GET[‘antiguoLANG’])){
$antID=$_GET[‘antiguoID’];
$antLANG=$_GET[‘antiguoLANG’];

$match=$wpdb->get_results(‘select det.post_id from cgn_detalles det where oldid=’.$antID);
if($match){
$newID=$match[0]->post_id;
if($antLANG!=’es’){
$newID=icl_object_id($newID, get_post_type($newID, false, $antLANG));
}
$npguid=get_permalink($newID);
header(“Location: “.$npguid, true, 301);
exit;
}else{
header(“HTTP/1.0 410 Gone”);
header(“Status: 410”);
echo “removed content”;
exit;
}
}

WPEngine suggests we go to a plugin.

Any suggestions ?