Startup.cs

// --------------------------------------------------------------------------------------------------------------------
// <copyright file="Startup.cs" company="AM">
//   Andre Mendonca
// </copyright>
// <summary>
//   The startup.
// </summary>
// --------------------------------------------------------------------------------------------------------------------

namespace LocalizationApi
{
    using System.Globalization;

    using Microsoft.AspNetCore.Builder;
    using Microsoft.AspNetCore.Hosting;
    using Microsoft.AspNetCore.Http;
    using Microsoft.AspNetCore.Localization;
    using Microsoft.Extensions.DependencyInjection;
    using Microsoft.Extensions.Hosting;
    using Microsoft.Extensions.Localization;
    using Microsoft.Extensions.Options;

    /// <summary>
    /// The startup.
    /// </summary>
    public class Startup
    {
        /// <summary>
        /// The configure services.
        /// </summary>
        /// <param name="services">
        /// The services.
        /// </param>
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddLocalization(options => options.ResourcesPath = "Resources");
            
            services
                .Configure<RequestLocalizationOptions>(options =>
                {
                    var cultures = new[]
                    {
                        new CultureInfo("en"),
                        new CultureInfo("pt")
                    };
                    options.DefaultRequestCulture = new RequestCulture("en");
                    options.SupportedCultures = cultures;
                    options.SupportedUICultures = cultures;
                });
        }

        /// <summary>
        /// The configure.
        /// </summary>
        /// <param name="app">
        /// The app.
        /// </param>
        /// <param name="env">
        /// The env.
        /// </param>
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

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

            app.UseEndpoints(endpoints =>
                {
                    endpoints.MapGet(
                        "/",
                        async context =>
                            {
                                await context.Response.WriteAsync(
                        context
                            .RequestServices
                            .GetRequiredService<IStringLocalizer<LocalizationResource>>()
                            .GetString("welcome"));
                });
            });
        }
    }
}