Add function get_user_lang for getting user language based on cookies

src/MD_STD.php

@@ -334,6 +334,45 @@ final class MD_STD {
 
     }
 
+    /**
+     * Sets and returns user language based on a session cookie.
+     *
+     * @param array<string> $allowed_langs Allowed languages.
+     * @param string        $default_lang  Default language.
+     *
+     * @return string
+     */
+    public function get_user_lang(array $allowed_langs, string $default_lang):string {
+
+        $cookie_options = [
+            'expires' => self::strtotime("+1 month"),
+            'path' => '/',
+            'domain' => $_SERVER['SERVER_NAME'],
+            'secure' => true,
+            'httponly' => true,
+            'samesite' => 'Lax' // None || Lax  || Strict
+        ];
+
+        if (isset($_GET['navlang']) and in_array($_GET['navlang'], $allowed_langs, true)) {
+            if (!(setcookie('lang', $_GET['navlang'], $cookie_options))) {
+                throw new Exception("Failed to set language");
+            }
+            $lang = $_GET['navlang'];
+        }
+        else if (isset($_COOKIE['lang']) and in_array($_COOKIE['lang'], $allowed_langs, true)) {
+            $lang = $_COOKIE['lang'];
+        }
+        else {
+            $lang = self::lang_getfrombrowser($allowed_langs, $default_lang, "", false);
+            if (!(setcookie('lang', $lang, $cookie_options))) {
+                throw new Exception("Failed to set language");
+            }
+        }
+
+        return $lang;
+
+    }
+
     /**
      * Function lang_getfrombrowser gets the browser language based on HTTP headers.
      *