When IPS4 is in developer mode language strings are loaded from the /dev/ directory inside each applications directory.
To edit or add the language strings, we do that via editing the file directly, not through the AdminCP.
Go to your /applications/YOURAPP/dev/lang.php, and you can add the new strings in this file:
$lang = array(
'__app_yourapp' => "YourApp",
'module__yourapp_yourapp' => "YourApp",