public class PersonalDetailsViewModel : OnboardingViewModel
{
  [RequiredLocalized]
  public string FirstName { get; set; }
  ....
}

public class RequiredLocalizedAttribute : ValidationAttribute, IClientValidatable
{
    public override string FormatErrorMessage(string name)
    {
        return " This field is mandatory";
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (base.IsValid(value))
        {
            return ValidationResult.Success;
        }
        return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        //string errorMessage = this.FormatErrorMessage(metadata.DisplayName);
        string errorMessage = ErrorMessageString;

        // The value we set here are needed by the jQuery adapter
        var requiredLocalizedRule = new ModelClientValidationRule
        {
            ErrorMessage = errorMessage,
            ValidationType = "requiredlocalized"
        };
        yield return requiredLocalizedRule;
    }
}

  jQuery.validator.unobtrusive.adapters.add("requiredlocalized", function (options) {
        options.rules["requiredlocalized"] = "true";
    });
    jQuery.validator.addMethod("requiredlocalized", function (value) {
        return value.length > 0;
    });

<input class="customText " data-val="true" data-val-length="Max length is 20 characters only"
data-val-length-max="20" data-val-requiredlocalized="The field {0} is invalid."
id="personalDetailsFirstName" maxlength="20" name="PersonalDetailsView.FirstName"
title="firstName" type="text" value="" />