Issue #3392199: Does not work on mono-lingual sites (when content is language neutral)

porterstemmer.module

@@ -42,7 +42,7 @@ function porterstemmer_search_preprocess($text, $langcode = NULL) {
     $langcode = \Drupal::languageManager()->getCurrentLanguage()->getId();
   }
 
-  if ($langcode == 'en') {
+  if ($langcode == 'en' || $langcode == 'und') {
     // Convert text to lower case, and replace special apostrophes with regular
     // apostrophes.
     $text = strtolower(str_replace('’', "'", $text));