Program.cs

using FastEndpoints;
using FastEndpoints.Swagger;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddAuthorization();
builder.Services.AddFastEndpoints()
               .SwaggerDocument(o =>
               {
                   o.MaxEndpointVersion = 1;
                   o.DocumentSettings = s =>
                   {
                       s.DocumentName = "V1";
                       s.Title = "API";
                       s.Version = "v1.0";
                   };
               }).AddLocalization(opt => opt.ResourcesPath = "Resources");


var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsProduction())
{
    app.UseSwaggerGen();
}

app.UseFastEndpoints(c =>
{
    c.Versioning.Prefix = "v";
    c.Versioning.PrependToRoute = true;
}).UseRequestLocalization(options =>
                    options
                    .AddSupportedCultures(new string[] { "en-US" })
                    .AddSupportedUICultures(new string[] { "en-US" })
                    .SetDefaultCulture("en-US")
            );


app.UseAuthorization();


app.Run();

namespace FastendpointsTest;

public class WeatherForecastRequest
{
    public int? CountryId { get; set; }

    public required DemographicData DemographicData { get; set; }
}

public class DemographicData
{
    public int? CountrySecret { get; set; }

    public string? CountryCode { get; set; }
}


using FastEndpoints;
using FluentValidation;
using Microsoft.Extensions.Localization;

namespace FastendpointsTest;

public class Validator : Validator<WeatherForecastRequest>
{


    public Validator(IStringLocalizer<SampleMessages> localizer)
    {
        RuleFor(x => x.CountryId).NotEmpty().WithMessage(localizer["Countryid"].Value);
        RuleFor(p => p.DemographicData).SetValidator(new DemographicDataValidator(localizer));
    }
}


public class DemographicDataValidator : Validator<DemographicData>
{
    public DemographicDataValidator()
    {

    }

    public DemographicDataValidator(IStringLocalizer<SampleMessages> localizer)
    {
    }
}