diff --git a/lib/classes/output/language_menu.php b/lib/classes/output/language_menu.php
index 846362bbe5e..a6cb9cf7376 100644
--- a/lib/classes/output/language_menu.php
+++ b/lib/classes/output/language_menu.php
@@ -151,7 +151,7 @@ class language_menu implements \renderable, \templatable {
             if ($langparam) {
                 $attributes = [
                     'data-lang' => $langparam,
-                    'lang' => $langparam,
+                    'lang' => get_html_lang_attribute_value($langparam),
                 ];
             }
             $lang = new \action_menu_link_secondary($node['url'], null, $node['title'], $attributes);
diff --git a/lib/tests/weblib_test.php b/lib/tests/weblib_test.php
index 98d25d2160f..4dc96ae6548 100644
--- a/lib/tests/weblib_test.php
+++ b/lib/tests/weblib_test.php
@@ -1163,9 +1163,10 @@ EXPECTED;
      */
     public function get_html_lang_attribute_value_provider() {
         return [
-            'Empty lang code' => ['    ', 'unknown'],
+            'Empty lang code' => ['    ', 'en'],
             'English' => ['en', 'en'],
-            'English, US' => ['en_us', 'en-us'],
+            'English, US' => ['en_us', 'en'],
+            'Unknown' => ['xx', 'en'],
         ];
     }
 
diff --git a/lib/weblib.php b/lib/weblib.php
index 733ea344b8a..41f9c629b44 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -2283,11 +2283,19 @@ function highlightfast($needle, $haystack) {
  * @return string
  */
 function get_html_lang_attribute_value(string $langcode): string {
-    if (empty(trim($langcode))) {
-        // If the language code passed is an empty string, return 'unknown'.
-        return 'unknown';
+    $langcode = clean_param($langcode, PARAM_LANG);
+    if ($langcode === '') {
+        return 'en';
     }
-    return str_replace('_', '-', $langcode);
+
+    // Grab language ISO code from lang config. If it differs from English, then it's been specified and we can return it.
+    $langiso = (string) (new lang_string('iso6391', 'core_langconfig', null, $langcode));
+    if ($langiso !== 'en') {
+        return $langiso;
+    }
+
+    // Where we cannot determine the value from lang config, use the first two characters from the lang code.
+    return substr($langcode, 0, 2);
 }
 
 /**