MDL-72882 core: Use default site lang when user lang no longer available

* Check that the lang attribute for the output HTML exists. If not,
use the default site language.
* Also fix the current language for the user.

lib/moodlelib.php

@@ -7161,6 +7161,39 @@ function current_language() {
     return $return;
 }
 
+/**
+ * Fix the current language to the given language code.
+ *
+ * @param string $lang The language code to use.
+ * @return void
+ */
+function fix_current_language(string $lang): void {
+    global $CFG, $COURSE, $SESSION, $USER;
+
+    if (!get_string_manager()->translation_exists($lang)) {
+        throw new coding_exception("The language pack for $lang is not available");
+    }
+
+    $fixglobal = '';
+    $fixlang = 'lang';
+    if (!empty($SESSION->forcelang)) {
+        $fixglobal = $SESSION;
+        $fixlang = 'forcelang';
+    } else if (!empty($COURSE->id) && $COURSE->id != SITEID && !empty($COURSE->lang)) {
+        $fixglobal = $COURSE;
+    } else if (!empty($SESSION->lang)) {
+        $fixglobal = $SESSION;
+    } else if (!empty($USER->lang)) {
+        $fixglobal = $USER;
+    } else if (isset($CFG->lang)) {
+        set_config('lang', $lang);
+    }
+
+    if ($fixglobal) {
+        $fixglobal->$fixlang = $lang;
+    }
+}
+
 /**
  * Returns parent language of current active language if defined
  *

lib/weblib.php

@@ -2233,6 +2233,16 @@ function get_html_lang_attribute_value(string $langcode): string {
  * @return string Attributes
  */
 function get_html_lang($dir = false) {
+    global $CFG;
+
+    $currentlang = current_language();
+    if ($currentlang !== $CFG->lang && !get_string_manager()->translation_exists($currentlang)) {
+        // Use the default site language when the current language is not available.
+        $currentlang = $CFG->lang;
+        // Fix the current language.
+        fix_current_language($currentlang);
+    }
+
     $direction = '';
     if ($dir) {
         if (right_to_left()) {
@@ -2241,8 +2251,9 @@ function get_html_lang($dir = false) {
             $direction = ' dir="ltr"';
         }
     }
+
     // Accessibility: added the 'lang' attribute to $direction, used in theme <html> tag.
-    $language = get_html_lang_attribute_value(current_language());
+    $language = get_html_lang_attribute_value($currentlang);
     @header('Content-Language: '.$language);
     return ($direction.' lang="'.$language.'" xml:lang="'.$language.'"');
 }