fix default handling of Request::i18n(), fixes #4774

Closes #4774

Merge request studip/studip!3566

lib/classes/Request.php

@@ -191,18 +191,18 @@ class Request implements ArrayAccess, IteratorAggregate
     /**
      * Return the value of the selected query parameter as an I18NString.
      *
-     * @param string   $param    parameter name
-     * @param string   $default  default value if parameter is not set
-     * @param Callable $op       Operation to perform on each text string
+     * @param string          $param   parameter name
+     * @param I18NString|null $default default value if parameter is not set
+     * @param callable|null   $op      Operation to perform on each text string
      *
      * @return I18NString  parameter value as string (if set), else NULL
      */
-    public static function i18n($param, $default = NULL, Callable $op = null)
+    public static function i18n(string $param, ?I18NString $default = NULL, Callable $op = null)
     {
-        $value = self::get($param, $default);
+        $value = self::get($param, $default ? $default->original() : null);
 
         if (isset($value)) {
-            $lang = self::getArray($param . '_i18n');
+            $lang = self::getArray($param . '_i18n') ?: ($default ? $default->toArray() : []);
 
             if ($op) {
                 $value = $op($value);