By default Zend Studio does not allow to change interface language to Russian. I found simple trick to do so. Go to the config folder (config_5.0 for Zend Studio 5.0 and config_5.1 for 5.1 version). This folder usually placed under c:Documents and Settings<Your Current User>ZDE.
Then open desktop_options.xml file and find following strings:
<locale language="en" country="" variant=""/>
You need just to change language attribute to ru and country attribute to RU. That’s All!