Typically, with client-side validation, the data attributes are controlled and handled by native MVC utilities. More specifically, by using the data annotation attributes. In your case, FluentValidation is a replacement of the data annotation attributes and the data attributes should be handled as an out of the box feature as it is said here—https://github.com/JeremySkinner/FluentValidation/wiki/h.-MVC
As the data attributes on the client are fetched from the server, client-side globalization and culture changes of the Kendo scripts are not part of the issue here.
The dates in the attributes are controlled entirely by the FluentValidation logic. That said, I suggest you checking whether the FluentValidtion takes properly the culture on the server and properly assigns the values for the data attributes on the server.
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