Update CKEditorHelper.php

Adding a fix language method due a incompatibility between translation tags used in CKEditor and Symfony. For example: symfony uses ``pt_BR`` to Brazilian Portuguese and CKeditor uses ``pt-br`` other languages as ``zh_CN`` will be positive affected too.
And can you add a test in https://github.com/egeloen/IvoryCKEditorBundle/blob/master/Tests/Templating/CKEditorHelperTest.php?