Unrecognized Locale "cz"

Hello, i have multi-language site with english and czech languages.

When i switch current language to czech, this code:

Yii::app()->dateFormatter->format('d MMMM y, H:m:s', $user->datereg);

generates error Unrecognized locale "cz"

Cause of the error is clear: there is no cz.php file in /framework/i18n/data, there is a file cs_cz.php

The question is how to solve this problem. I can’t directly change Yii::app()->locale