Add Content-Language header in localization middlewware

src/Middleware/Localization/Localization.slnf

@@ -0,0 +1,16 @@
+{
+  "solution": {
+    "path": "..\\Middleware.sln",
+    "projects": [
+      "Localization.Routing\\src\\Microsoft.AspNetCore.Localization.Routing.csproj",
+      "Localization.Routing\\test\\Microsoft.AspNetCore.Localization.Routing.Tests.csproj",
+      "Localization\\sample\\LocalizationSample.csproj",
+      "Localization\\src\\Microsoft.AspNetCore.Localization.csproj",
+      "Localization\\test\\FunctionalTests\\Microsoft.AspNetCore.Localization.FunctionalTests.csproj",
+      "Localization\\test\\UnitTests\\Microsoft.AspNetCore.Localization.Tests.csproj",
+      "Localization\\testassets\\LocalizationWebsite\\LocalizationWebsite.csproj",
+      "Localization\\testassets\\ResourcesClassLibraryNoAttribute\\ResourcesClassLibraryNoAttribute.csproj",
+      "Localization\\testassets\\ResourcesClassLibraryWithAttribute\\ResourcesClassLibraryWithAttribute.csproj"
+    ]
+  }
+}

src/Middleware/Localization/ref/Microsoft.AspNetCore.Localization.netcoreapp3.0.cs

@@ -13,6 +13,7 @@ public static partial class ApplicationBuilderExtensions
     public partial class RequestLocalizationOptions
     {
         public RequestLocalizationOptions() { }
+        public bool ApplyCurrentCultureToResponseHeaders { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
         public Microsoft.AspNetCore.Localization.RequestCulture DefaultRequestCulture { get { throw null; } set { } }
         public bool FallBackToParentCultures { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
         public bool FallBackToParentUICultures { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }

src/Middleware/Localization/src/RequestLocalizationMiddleware.cs

@@ -1,5 +1,5 @@
-// Copyright (c) .NET Foundation. All rights reserved. 
-// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information. 
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
 using System.Collections.Generic;
@@ -13,6 +13,7 @@
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using Microsoft.Extensions.Primitives;
+using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.Localization
 {
@@ -146,6 +147,11 @@ public async Task Invoke(HttpContext context)
 
             SetCurrentThreadCulture(requestCulture);
 
+            if (_options.ApplyCurrentCultureToResponseHeaders)
+            {
+                context.Response.Headers.Add(HeaderNames.ContentLanguage, requestCulture.UICulture.Name);
+            }
+
             await _next(context);
         }
 

src/Middleware/Localization/src/RequestLocalizationOptions.cs

@@ -84,6 +84,11 @@ public RequestCulture DefaultRequestCulture
         /// </example>
         public bool FallBackToParentUICultures { get; set; } = true;
 
+        /// <summary>
+        /// Gets or sets a value that determines if <see cref="CultureInfo.CurrentUICulture" /> is applied to the response <c>Content-Language</c> header.
+        /// </summary>
+        public bool ApplyCurrentCultureToResponseHeaders { get; set; }
+
         /// <summary>
         /// The cultures supported by the application. The <see cref="RequestLocalizationMiddleware"/> will only set
         /// the current request culture to an entry in this list.

src/Middleware/Localization/test/FunctionalTests/LocalizationTest.cs

@@ -1,5 +1,5 @@
-// Copyright (c) .NET Foundation. All rights reserved. 
-// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information. 
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
 using System.Net;
@@ -14,6 +14,15 @@ namespace Microsoft.AspNetCore.Localization.FunctionalTests
 {
     public class LocalizationTest
     {
+        [Fact]
+        public Task Localization_ContentLanguageHeader()
+        {
+            return RunTest(
+                typeof(StartupContentLanguageHeader),
+                "ar-YE",
+                "True ar-YE");
+        }
+
         [Fact]
         public Task Localization_CustomCulture()
         {

src/Middleware/Localization/testassets/LocalizationWebsite/StartupContentLanguageHeader.cs

@@ -0,0 +1,49 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using System.Globalization;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Localization;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Net.Http.Headers;
+
+namespace LocalizationWebsite
+{
+    public class StartupContentLanguageHeader
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddLocalization();
+        }
+
+        public void Configure(
+            IApplicationBuilder app)
+        {
+            app.UseRequestLocalization(new RequestLocalizationOptions
+            {
+                DefaultRequestCulture = new RequestCulture("en-US"),
+                SupportedCultures = new List<CultureInfo>()
+                {
+                    new CultureInfo("ar-YE")
+                },
+                SupportedUICultures = new List<CultureInfo>()
+                {
+                    new CultureInfo("ar-YE")
+                },
+                ApplyCurrentCultureToResponseHeaders = true
+            });
+
+            app.Run(async (context) =>
+            {
+                var hasContentLanguageHeader = context.Response.Headers.ContainsKey(HeaderNames.ContentLanguage);
+                var contentLanguage = context.Response.Headers[HeaderNames.ContentLanguage].ToString();
+
+                await context.Response.WriteAsync(hasContentLanguageHeader.ToString());
+                await context.Response.WriteAsync(" ");
+                await context.Response.WriteAsync(contentLanguage);
+            });
+        }
+    }
+}