Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Options;

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

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            var cultures = new[]
            {
                "de-CH", // Deutsch "Sie" / German formal
                "de-CH-x-informal", // Deutsch "du" / German informal
                "fr-CH", // français "vous" / French formal
                "fr-CH-x-informal", // français "tu" / French informal
                "en-CH" // English
            };
            services.Configure<RequestLocalizationOptions>(options => options
            .AddSupportedCultures(cultures)
            .AddSupportedUICultures(cultures));
            services.AddLocalization();
            services.AddRazorPages()
                .AddViewLocalization(LanguageViewLocationExpanderFormat.Suffix, x => x.ResourcesPath = "Resources");
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Error");
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();

            app.UseRouting();
            app.UseRequestLocalization(app.ApplicationServices.GetService<IOptions<RequestLocalizationOptions>>().Value);
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapRazorPages();
            });
        }
    }
}