These messages are built in the default FileSystemContentProvider and they are not exposed in the Localization file. In this case I recommend you to change the strings in the provider directly. For your convenience I have attached a project with the configured new provider. In the project I used the default provider FileSystemContentProvider
, and changed its class name to CustomFileSystemContentProvider.
You could find the text that you need to change in the provider and replace it with the desired one.
I hope this helps.
the Telerik team