How do I debug/kint Simple XML Sitemap link array?
As Clive said, you will need to make sure that both the devel and kint modules are enabled.  (NOTE: D8's Devel 4.0 now includes the kint module but requires 'composer require kint-php/kint' to be done to make it work) Then you can add your hook and just do a ksm($links); statement.
Make sure to clear cache so the hook will be found.  Then doing a regenerate of the sitemap(s) in the gui will product the kint output that you can examine for the structure.
Also, not totally sure, but I think this hook has to be in a module and will not work in the .theme file.
FYI - Here's a code snippet from a working hook that does what you want, including removing untranslated nodes.
/**
 * Implements hook_simple_sitemap_links_alter().
 */
function my_module_simple_sitemap_links_alter(&amp;$links, $variant) {
  foreach($links as $key =&gt; $link) {
    // Filter out any node/### URLs
    if ( preg_match(&quot;!(/[a-z-]*)?/node/\d?!&quot;, $link['url']) ) {
      unset( $links[$key] );
      continue;
    }
}