This guide is for more or less advanced users. If you just started with Yii or didn't worked with any web-framework before, you may should come back later :-)

As seen in this post, Yii doesn't enforce how language is set and maintained within the session.

Yii core messages refer to static text strings in the core Yii framework code which are meant to be displayed to end-users (e.g. core exception messages, default validation error messages). Customization of these core messages is needed in two circumstances:

To fix issues with display of special language characters once and for all there's a solution: use Unicode UTF-8 everywhere. If everything is set up to use Unicode, you can use mostly every language in your application.