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.