Hi,

Could you please try to follow the instruction mentioned here - http://www.kentico.com/docs/devguide/international_support.htm

You need to change the default strings directly in the .resx file or, the recommended way is that you will find appropriate keys in the resx file and then add your custom strings with the same keys into to Site Manager section as mentioned in the link.

Best Regards,
Juraj Ondrus