implement JsonSerializable for I18NString, fixes #1672

Closes #1672

Merge request studip/studip!1076

lib/classes/I18NString.php

@@ -3,9 +3,8 @@
 /**
  * I18NString class
  */
-class I18NString
+class I18NString implements JsonSerializable
 {
-
     /**
      * Text in default content language.
      *
@@ -77,6 +76,16 @@ class I18NString
         return (string) $this->base;
     }
 
+    /**
+     * Return the JSON representation of this i18n field in selected language.
+     *
+     * @return string
+     */
+    public function jsonSerialize()
+    {
+        return (string) $this;
+    }
+
     /**
      * Sets the language the content is translated into.
      *