diff --git a/h5p/classes/editor_framework.php b/h5p/classes/editor_framework.php
index dc4e4216990..0ab8b956338 100644
--- a/h5p/classes/editor_framework.php
+++ b/h5p/classes/editor_framework.php
@@ -84,7 +84,7 @@ class editor_framework implements H5peditorStorage {
 
             // If current language has a dependency, then request it.
             if (count($dependencies) > 1) {
-                $parentlanguage = str_replace('_', '-', $dependencies[count($dependencies) - 2]);
+                $parentlanguage = get_html_lang_attribute_value($dependencies[count($dependencies) - 2]);
                 $result = $this->get_language_record($name, $major, $minor, $parentlanguage);
             }
         }
diff --git a/h5p/classes/framework.php b/h5p/classes/framework.php
index 4c1241b83d1..c43fe95f1f6 100644
--- a/h5p/classes/framework.php
+++ b/h5p/classes/framework.php
@@ -1610,7 +1610,7 @@ class framework implements H5PFrameworkInterface {
         }
 
         // Get current language in Moodle.
-        $language = str_replace('_', '-', strtolower(\current_language()));
+        $language = get_html_lang_attribute_value(strtolower(\current_language()));
 
         // Try to map.
         return $map[$language] ?? $language;
diff --git a/lib/classes/content/export/zipwriter.php b/lib/classes/content/export/zipwriter.php
index 438b2e32aa8..5666cf3c83d 100644
--- a/lib/classes/content/export/zipwriter.php
+++ b/lib/classes/content/export/zipwriter.php
@@ -255,7 +255,7 @@ class zipwriter {
 
         $templatedata->global = (object) [
             'righttoleft' => right_to_left(),
-            'language' => str_replace('_', '-', current_language()),
+            'language' => get_html_lang_attribute_value(current_language()),
             'sitename' => format_string($SITE->fullname, true, ['context' => context_system::instance()]),
             'siteurl' => $CFG->wwwroot,
             'pathtotop' => $this->get_relative_context_path($context, $this->rootcontext, '/'),
diff --git a/lib/classes/output/language_menu.php b/lib/classes/output/language_menu.php
index 36384725a18..846362bbe5e 100644
--- a/lib/classes/output/language_menu.php
+++ b/lib/classes/output/language_menu.php
@@ -96,6 +96,14 @@ class language_menu implements \renderable, \templatable {
         // Add the lang picker if needed.
         foreach ($this->langs as $langtype => $langname) {
             $isactive = $langtype == $this->currentlang;
+            $attributes = [];
+            if (!$isactive) {
+                // Set the lang attribute for languages different from the page's current language.
+                $attributes[] = [
+                    'key' => 'lang',
+                    'value' => get_html_lang_attribute_value($langtype),
+                ];
+            }
             $node = [
                 'title' => $langname,
                 'text' => $langname,
@@ -103,6 +111,9 @@ class language_menu implements \renderable, \templatable {
                 'isactive' => $isactive,
                 'url' => $isactive ? new \moodle_url('#') : new \moodle_url($this->page->url, ['lang' => $langtype]),
             ];
+            if (!empty($attributes)) {
+                $node['attributes'] = $attributes;
+            }
 
             $nodes[] = $node;
 
@@ -135,8 +146,15 @@ class language_menu implements \renderable, \templatable {
         }
         $langmenu->set_menu_trigger($menuname);
         foreach ($languagedata['items'] as $node) {
-            $lang = new \action_menu_link_secondary($node['url'], null, $node['title'],
-                ['data-lang' => $node['url']->get_param('lang')]);
+            $langparam = $node['url']->get_param('lang');
+            $attributes = [];
+            if ($langparam) {
+                $attributes = [
+                    'data-lang' => $langparam,
+                    'lang' => $langparam,
+                ];
+            }
+            $lang = new \action_menu_link_secondary($node['url'], null, $node['title'], $attributes);
             $langmenu->add($lang);
         }
         return $langmenu->export_for_template($output);
diff --git a/lib/outputcomponents.php b/lib/outputcomponents.php
index 613ef61dbab..c1b22e185f4 100644
--- a/lib/outputcomponents.php
+++ b/lib/outputcomponents.php
@@ -3487,6 +3487,9 @@ class custom_menu_item implements renderable, templatable {
      */
     protected $lastsort = 0;
 
+    /** @var array Array of other HTML attributes for the custom menu item. */
+    protected $attributes = [];
+
     /**
      * Constructs the new custom menu item
      *
@@ -3496,13 +3499,16 @@ class custom_menu_item implements renderable, templatable {
      * @param int $sort A sort or to use if we need to sort differently [Optional]
      * @param custom_menu_item $parent A reference to the parent custom_menu_item this child
      *        belongs to, only if the child has a parent. [Optional]
+     * @param array $attributes Array of other HTML attributes for the custom menu item.
      */
-    public function __construct($text, moodle_url $url=null, $title=null, $sort = null, custom_menu_item $parent = null) {
+    public function __construct($text, moodle_url $url = null, $title = null, $sort = null, custom_menu_item $parent = null,
+                                array $attributes = []) {
         $this->text = $text;
         $this->url = $url;
         $this->title = $title;
         $this->sort = (int)$sort;
         $this->parent = $parent;
+        $this->attributes = $attributes;
     }
 
     /**
@@ -3512,14 +3518,15 @@ class custom_menu_item implements renderable, templatable {
      * @param moodle_url $url
      * @param string $title
      * @param int $sort
+     * @param array $attributes Array of other HTML attributes for the custom menu item.
      * @return custom_menu_item
      */
-    public function add($text, moodle_url $url = null, $title = null, $sort = null) {
+    public function add($text, moodle_url $url = null, $title = null, $sort = null, $attributes = []) {
         $key = count($this->children);
         if (empty($sort)) {
             $sort = $this->lastsort + 1;
         }
-        $this->children[$key] = new custom_menu_item($text, $url, $title, $sort, $this);
+        $this->children[$key] = new custom_menu_item($text, $url, $title, $sort, $this, $attributes);
         $this->lastsort = (int)$sort;
         return $this->children[$key];
     }
@@ -3652,8 +3659,15 @@ class custom_menu_item implements renderable, templatable {
         $context = new stdClass();
         $context->text = external_format_string($this->text, $syscontext->id);
         $context->url = $this->url ? $this->url->out() : null;
-        $context->title = external_format_string($this->title, $syscontext->id);
+        // No need for the title if it's the same with text.
+        if ($this->text !== $this->title) {
+            // Show the title attribute only if it's different from the text.
+            $context->title = external_format_string($this->title, $syscontext->id);
+        }
         $context->sort = $this->sort;
+        if (!empty($this->attributes)) {
+            $context->attributes = $this->attributes;
+        }
         $context->children = array();
         if (preg_match("/^#+$/", $this->text)) {
             $context->divider = true;
diff --git a/lib/outputrenderers.php b/lib/outputrenderers.php
index e86201b8015..f8efbe6e4f0 100644
--- a/lib/outputrenderers.php
+++ b/lib/outputrenderers.php
@@ -3823,13 +3823,21 @@ EOD;
             $strlang = get_string('language');
             $currentlang = current_language();
             if (isset($langs[$currentlang])) {
-                $currentlang = $langs[$currentlang];
+                $currentlangstr = $langs[$currentlang];
             } else {
-                $currentlang = $strlang;
+                $currentlangstr = $strlang;
             }
-            $this->language = $menu->add($currentlang, new moodle_url('#'), $strlang, 10000);
+            $this->language = $menu->add($currentlangstr, new moodle_url('#'), $strlang, 10000);
             foreach ($langs as $langtype => $langname) {
-                $this->language->add($langname, new moodle_url($this->page->url, array('lang' => $langtype)), $langname);
+                $attributes = [];
+                // Set the lang attribute for languages different from the page's current language.
+                if ($langtype !== $currentlang) {
+                    $attributes[] = [
+                        'key' => 'lang',
+                        'value' => get_html_lang_attribute_value($langtype),
+                    ];
+                }
+                $this->language->add($langname, new moodle_url($this->page->url, ['lang' => $langtype]), null, null, $attributes);
             }
         }
 
diff --git a/lib/templates/custom_menu_item.mustache b/lib/templates/custom_menu_item.mustache
index 5bff2f9757d..006da02e015 100644
--- a/lib/templates/custom_menu_item.mustache
+++ b/lib/templates/custom_menu_item.mustache
@@ -39,7 +39,7 @@
     <div class="dropdown-menu" role="menu" id="drop-down-menu-{{uniqid}}" aria-labelledby="drop-down-{{uniqid}}">
         {{#children}}
             {{^divider}}
-                <a class="dropdown-item" role="menuitem" href="{{{url}}}" {{#title}}title="{{{title}}}"{{/title}}>{{{text}}}</a>
+                <a class="dropdown-item" role="menuitem" href="{{{url}}}" {{#title}}title="{{{title}}}"{{/title}} {{#attributes}}{{key}}="{{value}}" {{/attributes}}>{{{text}}}</a>
             {{/divider}}
             {{#divider}}
                 <div class="dropdown-divider" role="presentation"></div>
diff --git a/lib/templates/user_action_menu_submenu_items.mustache b/lib/templates/user_action_menu_submenu_items.mustache
index 605ab867116..35a25e3eeea 100644
--- a/lib/templates/user_action_menu_submenu_items.mustache
+++ b/lib/templates/user_action_menu_submenu_items.mustache
@@ -41,7 +41,8 @@
 }}
 {{#items}}
     {{#link}}
-        <a href="{{{url}}}" class="dropdown-item pl-5" role="menuitem" tabindex="-1" {{#isactive}}aria-current="true"{{/isactive}}>
+        <a href="{{{url}}}" class="dropdown-item pl-5" role="menuitem" tabindex="-1" {{#isactive}}aria-current="true"{{/isactive}}
+            {{#attributes}}{{key}}="{{value}}" {{/attributes}}>
             {{text}}
         </a>
     {{/link}}
diff --git a/lib/tests/output/language_menu_test.php b/lib/tests/output/language_menu_test.php
index eccf12e974e..7587227f8f3 100644
--- a/lib/tests/output/language_menu_test.php
+++ b/lib/tests/output/language_menu_test.php
@@ -72,10 +72,19 @@ class language_menu_test extends \advanced_testcase {
             // Assert that the number of language menu items matches the number of the expected items.
             $this->assertEquals(count($expected['items']), count($response['items']));
             foreach ($expected['items'] as $expecteditem) {
+                $lang = $expecteditem['lang'];
                 // We need to manually generate the url key and its value in the expected item array as this cannot
                 // be done in the data provider due to the change of the state of $PAGE.
-                $expecteditem['url'] = $expecteditem['isactive'] ? new \moodle_url('#') :
-                    new \moodle_url($PAGE->url, ['lang' => $expecteditem['lang']]);
+                if ($expecteditem['isactive']) {
+                    $expecteditem['url'] = new \moodle_url('#');
+                } else {
+                    $expecteditem['url'] = new \moodle_url($PAGE->url, ['lang' => $lang]);
+                    // When the language menu item is not the current language, it will contain the lang attribute.
+                    $expecteditem['attributes'][] = [
+                        'key' => 'lang',
+                        'value' => $lang
+                    ];
+                }
                 // The lang value is only used to generate the url, so this key can be removed.
                 unset($expecteditem['lang']);
 
diff --git a/lib/tests/weblib_test.php b/lib/tests/weblib_test.php
index 8fa7d6a9924..a6c6aa9b3c3 100644
--- a/lib/tests/weblib_test.php
+++ b/lib/tests/weblib_test.php
@@ -941,4 +941,30 @@ EXPECTED;
 
         $this->assertNotEquals($policydisabled, print_password_policy());
     }
+
+    /**
+     * Data provider for the testing get_html_lang_attribute_value().
+     *
+     * @return string[][]
+     */
+    public function get_html_lang_attribute_value_provider() {
+        return [
+            'Empty lang code' => ['    ', 'unknown'],
+            'English' => ['en', 'en'],
+            'English, US' => ['en_us', 'en-us'],
+        ];
+    }
+
+    /**
+     * Test for get_html_lang_attribute_value().
+     *
+     * @covers ::get_html_lang_attribute_value()
+     * @dataProvider get_html_lang_attribute_value_provider
+     * @param string $langcode The language code to convert.
+     * @param string $expected The expected converted value.
+     * @return void
+     */
+    public function test_get_html_lang_attribute_value(string $langcode, string $expected): void {
+        $this->assertEquals($expected, get_html_lang_attribute_value($langcode));
+    }
 }
diff --git a/lib/weblib.php b/lib/weblib.php
index 7b142723eed..ea7a294bf27 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -2206,6 +2206,24 @@ function highlightfast($needle, $haystack) {
     return str_replace('<span class="highlight"></span>', '', join('', $parts));
 }
 
+/**
+ * Converts a language code to hyphen-separated format in accordance to the
+ * {@link https://datatracker.ietf.org/doc/html/rfc5646#section-2.1 BCP47 syntax}.
+ *
+ * For additional information, check out
+ * {@link https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang MDN web docs - lang}.
+ *
+ * @param string $langcode The language code to convert.
+ * @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';
+    }
+    return str_replace('_', '-', $langcode);
+}
+
 /**
  * Return a string containing 'lang', xml:lang and optionally 'dir' HTML attributes.
  *
@@ -2224,7 +2242,7 @@ function get_html_lang($dir = false) {
         }
     }
     // Accessibility: added the 'lang' attribute to $direction, used in theme <html> tag.
-    $language = str_replace('_', '-', current_language());
+    $language = get_html_lang_attribute_value(current_language());
     @header('Content-Language: '.$language);
     return ($direction.' lang="'.$language.'" xml:lang="'.$language.'"');
 }
diff --git a/theme/boost/templates/language_menu.mustache b/theme/boost/templates/language_menu.mustache
index 305f8a85c63..b3f77d2d9e7 100644
--- a/theme/boost/templates/language_menu.mustache
+++ b/theme/boost/templates/language_menu.mustache
@@ -53,7 +53,8 @@
         <div role="menu" aria-labelledby="lang-menu-toggle" id="lang-action-menu" class="dropdown-menu dropdown-menu-right">
             {{#items}}
                 {{#link}}
-                    <a href="{{{url}}}" class="dropdown-item pl-5" role="menuitem" {{#isactive}}aria-current="true"{{/isactive}}>
+                    <a href="{{{url}}}" class="dropdown-item pl-5" role="menuitem" {{#isactive}}aria-current="true"{{/isactive}}
+                            {{#attributes}}{{key}}="{{value}}" {{/attributes}}>
                         {{text}}
                     </a>
                 {{/link}}