Startup.cs

using Localization.Misc;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Localization;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using System.Collections.Generic;
using System.Globalization;

namespace Localization
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddSingleton<MyLocalizer>();

            services.AddLocalization(options => options.ResourcesPath = "Resources");

            var supportedCultures = new List<CultureInfo>() {
                new CultureInfo("en-US"),
                new CultureInfo("de-DE"),
                new CultureInfo("fr-FR")
            };
            services.Configure<RequestLocalizationOptions>(options =>
            {
                options.DefaultRequestCulture = new RequestCulture("en-US");
                options.SupportedCultures = supportedCultures;
                options.SupportedUICultures = supportedCultures;
                options.RequestCultureProviders.Clear(); // Clears all the default culture providers from the list
                options.RequestCultureProviders.Add(new MyCultureProvider()); // Add custom culture provider
            });

            services.AddControllersWithViews();
            services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme).AddCookie(c => c.LoginPath = "/home/index");
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseStaticFiles();

            app.UseRouting();

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseRequestLocalization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
            });
        }
    }
}