diff --git a/filter/multilang/filter.php b/filter/multilang/filter.php
index 5dcd572ef0a..2e9bac35575 100644
--- a/filter/multilang/filter.php
+++ b/filter/multilang/filter.php
@@ -40,6 +40,8 @@ defined('MOODLE_INTERNAL') || die();
 
 
 /**
+ * Implementation of the Moodle filter API for the Multi-lang filter.
+ *
  * @copyright  Gaetan Frenoy <gaetan@frenoy.net>
  * @copyright  2004 onwards Eloy Lafuente (stronk7) {@link http://stronk7.com}
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
@@ -81,38 +83,52 @@ class filter_multilang extends moodle_text_filter {
      * @return string the replacement string (one of the possible translations).
      */
     protected function process_match(array $langblock): string {
-        $mylang = current_language();
-        static $parentcache;
-        if (!isset($parentcache)) {
-            $parentcache = array();
-        }
-        if (!array_key_exists($mylang, $parentcache)) {
-            $parentlang = get_parent_language($mylang);
-            $parentcache[$mylang] = $parentlang;
-        } else {
-            $parentlang = $parentcache[$mylang];
-        }
-
         $searchtosplit = '/<(?:lang|span)[^>]+lang="([a-zA-Z0-9_-]+)"[^>]*>(.*?)<\/(?:lang|span)>/is';
 
         if (!preg_match_all($searchtosplit, $langblock[0], $rawlanglist)) {
-            //skip malformed blocks
+            // Skip malformed blocks.
             return $langblock[0];
         }
 
         $langlist = array();
-        foreach ($rawlanglist[1] as $index=>$lang) {
-            $lang = str_replace('-','_',strtolower($lang)); // normalize languages
+        foreach ($rawlanglist[1] as $index => $lang) {
+            $lang = str_replace('-', '_', strtolower($lang)); // Normalize languages.
             $langlist[$lang] = $rawlanglist[2][$index];
         }
 
-        if (array_key_exists($mylang, $langlist)) {
-            return $langlist[$mylang];
-        } else if (array_key_exists($parentlang, $langlist)) {
-            return $langlist[$parentlang];
-        } else {
-            $first = array_shift($langlist);
-            return $first;
+        // Follow the stream of parent languages.
+        $lang = current_language();
+        do {
+            if (isset($langlist[$lang])) {
+                return $langlist[$lang];
+            }
+        } while ($lang = $this->get_parent_lang($lang));
+
+        // If we don't find a match, default to the first provided translation.
+        return array_shift($langlist);
+    }
+
+    /**
+     * Puts some caching around get_parent_language().
+     *
+     * Also handle parent == 'en' in a way that works better for us.
+     *
+     * @param string $lang a Moodle language code, e.g. 'fr'.
+     * @return string the parent language.
+     */
+    protected function get_parent_lang(string $lang): string {
+        static $parentcache;
+        if (!isset($parentcache)) {
+            $parentcache = ['en' => ''];
         }
+        if (!isset($parentcache[$lang])) {
+            $parentcache[$lang] = get_parent_language($lang);
+            // The standard get_parent_language method returns '' for parent == 'en'.
+            // That is less helpful for us, so change it back.
+            if ($parentcache[$lang] === '') {
+                $parentcache[$lang] = 'en';
+            }
+        }
+        return $parentcache[$lang];
     }
 }
diff --git a/filter/multilang/tests/filter_test.php b/filter/multilang/tests/filter_test.php
index 116dc50f02a..2dbefa2c821 100644
--- a/filter/multilang/tests/filter_test.php
+++ b/filter/multilang/tests/filter_test.php
@@ -116,6 +116,11 @@ class filter_multilang_filter_testcase extends advanced_testcase {
                 <span lang="fr_ca" class="multilang">Québécois</span>',
                 'fr', ['fr_ca' => 'fr'],
             ],
+            'Fallback to parent when child not present when parent is en' => [
+                'English',
+                '<span lang="de" class="multilang">Deutsch</span><span lang="en" class="multilang">English</span>',
+                'en_us',
+            ],
         ];
     }