FIle AdminCustomCodeController.php, #73 line has code:
'description' => $this->l('On this page you can add extra HTML between the <head> tags, extra CSS or JavaScript to your pages. JavaScript should NOT be enclosed with <script> tags. ...
Note, that < > tags can't be included in the translation string.
This bug prevent to translate thirty bees back-office into other language. Translation form stops on this translation string.