Thank you for contacting Syncfusion support.
By default, there is no culture support for validation messages but the content can be customized as shown below. Kindly use your required culture message for the respective fields.
<Code>
.Subject(subject => subject.Field("Subject").ValidationRules(v => v.AddRule("required", true).AddRule("messages", new Dictionary<string, object>() { { "required", "Specify your culture message." } })))
</Code>
Regards,
Karthigeyan