I just recognized by incident that the great Ultimate Tag Warrior Plugin by Christine Davis doesn’t take German Umlauts into account correctly under certain circumstances, e.g. if you utilize the “related posts” feature like Binary Blue does (if configured this way). So I dug into the code of the core files of my currently installed 3.1 release of that plugin and found the appropriate function to modify. The mod you need to apply is pretty simple - I just added a few lines of code to make use of a hopefully installed and activated o42-Clean-Umlauts plugin (or its 4null4.de pendant):

function FormatPost($post, $format) {
  $URL = get_permalink($post->ID);

  // CleanUmlauts-Integration
  if (function_exists('o42_cu_content'))
  {
    $post->post_title   = o42_cu_content($post->post_title);
    $post->post_excerpt = o42_cu_content($post->post_excerpt);
    $post->post_content = o42_cu_content($post->post_content);
  }

  $format = str_replace('%title%', $post->post_title, $format);
  $format = str_replace('%postlink%', '<a href="'.$URL.'">'.$post->post_title.'</a>', $format);
  $format = str_replace('%excerpt%', $post->post_excerpt, $format);
  $format = str_replace('%postdate%',mysql2date(get_settings("date_format"), $post->post_date),$format);
  $format = str_replace('%content%', $post->post_content, $format);

  return $format;
}

To add appropriate support for German Umlauts in their correct character encoding, just replace the original FormatPost()-function in the core file of UTW with the code displayed above and reupload that file to your webserver, overwriting the original file in your UTW plugin directory - that’s it, full and correct German Umlaut support for the Related Posts lists.

Additionally, you can add full support for tags containing German Umlauts as well. Just do the following:

// original: $tag_name = strtolower($tag->tag);
$tag_name = utf8_decode(strtolower(htmlentities(utf8_decode($tag->tag))));
// original: $tag_name_url = urlencode(stripslashes($tag_name));
$tag_name_url = urlencode(stripslashes(strtolower($tag->tag)));

Look for the codelines 1092/1093 in the core file - it’s the two lines of code in the sample above I’ve commented out. Replace them with the code displayed above, reupload yadda yadda, and everything will be fine - both display of the tags as well as the distinct tag archives as well as the tag cloud will work perfectly then with german Umlauts as well.