PM> Install-Package AspNetCore.Mvc.UrlLocalization

> dotnet add package AspNetCore.Mvc.UrlLocalization

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

	public IConfiguration Configuration { get; }

	public bool RedirectCulturelessToDefaultCulture = false;

	// This method gets called by the runtime. Use this method to add services to the container.
	public void ConfigureServices(IServiceCollection services)
	{
		services.AddLocalization(options => options.ResourcesPath = "Resources");
		services.AddUrlLocalization();

		services.AddCultureRouteConstraint("cultureCheck");

		services.AddControllersWithViews(options =>
		{
			//Adds {culture:cultureCheck} to ALL routes
			if (RedirectCulturelessToDefaultCulture)
				options.AddCultureAttributeRouteConvention("culture", "cultureCheck");
			else
				options.AddOptionalCultureAttributeRouteConvention("culture", "cultureCheck");

			//options.Filters.Add(new MvcUrlLocalizationFilterAttribute());
		})
		.AddRazorPagesOptions(options => {
			if (RedirectCulturelessToDefaultCulture)
				options.AddCultureAttributeRouteConvention();
			else
				options.AddOptionalCultureAttributeRouteConvention();
		})
		.AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix)
		.AddDataAnnotationsLocalization()
		.AddActionLinkLocalization()
		.AddAmbientRouteDataUrlHelperFactory(options =>
		{
			options.AmbientRouteDataKeys.Add(new AmbientRouteDataKey("area", false));
			options.AmbientRouteDataKeys.Add(new AmbientRouteDataKey("culture", true));
			options.AmbientRouteDataKeys.Add(new AmbientRouteDataKey("ui-culture", true));
		});

		var supportedCultures = new[]
		 {
			new CultureInfo("en-US"),
			new CultureInfo("es"),
			new CultureInfo("fr"),
			new CultureInfo("de")
		};

		services.Configure<RequestLocalizationOptions>(options =>
		{
			options.DefaultRequestCulture = new RequestCulture("en-US");
			// Formatting numbers, dates, etc.
			options.SupportedCultures = supportedCultures;
			// UI strings that we have localized.
			options.SupportedUICultures = supportedCultures;
			options.RequestCultureProviders = new List<IRequestCultureProvider>()
			{
				 new RouteDataRequestCultureProvider() { Options = options, RouteDataStringKey = "culture", UIRouteDataStringKey = "ui-culture" },
				 new UrlRequestCultureProvider(),
				 new QueryStringRequestCultureProvider() { QueryStringKey = "culture", UIQueryStringKey = "ui-culture" },
				 new CookieRequestCultureProvider(),
				 new AcceptLanguageHeaderRequestCultureProvider(),
			};
		});

		services.AddSingleton(sp => sp.GetService<IOptions<RequestLocalizationOptions>>().Value);

		services.Configure<RedirectUnsupportedUrlCulturesOptions>(options =>
		{
			options.RedirectUnspportedCulturesToDefaultCulture = true;
			options.RedirectCulturelessToDefaultCulture = RedirectCulturelessToDefaultCulture;
		});

		services.AddSingleton(sp => sp.GetService<IOptions<RedirectUnsupportedUrlCulturesOptions>>().Value);

		services.Configure<RouteOptions>(options => options.LowercaseUrls = true);
	}

	// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
	public void Configure(IApplicationBuilder app, IWebHostEnvironment env, RequestLocalizationOptions localizationOptions, RedirectUnsupportedUrlCulturesOptions redirectUnsupportedUrlCulturesOptions)
	{
		if (env.IsDevelopment())
		{
			app.UseDeveloperExceptionPage();
		}
		else
		{
			app.UseExceptionHandler("/Home/Error");
			// The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
			app.UseHsts();
		}
		app.UseHttpsRedirection();        

		app.UseStaticFiles();

		app.UseRequestLocalization(localizationOptions);
		app.UseRedirectUnsupportedUrlCultures(redirectUnsupportedUrlCulturesOptions);
		app.UseUrlUnlocalization();

		app.UseRouting();

		app.UseAuthorization();

		app.UseEndpoints(endpoints =>
		{
			if (redirectUnsupportedUrlCulturesOptions.RedirectCulturelessToDefaultCulture)
			{
				endpoints.MapControllerRoute(
				 name: "defaultWithCulture",
				 pattern: "{culture:cultureCheck}/{controller=Home}/{action=Index}/{id?}");

				//Other Routes

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

				endpoints.MapControllerRoute(
				 name: "defaultWithCulture",
				 pattern: "{culture:cultureCheck}/{controller=Home}/{action=Index}/{id?}");

				//Other Routes
			}
		});
	}
}