Allow CLI UI language to be overridden by an environment variable

src/dotnet/Program.cs

@@ -198,6 +198,9 @@ namespace Microsoft.DotNet.Cli
             // by default, .NET Core doesn't have all code pages needed for Console apps.
             // see the .NET Core Notes in https://msdn.microsoft.com/en-us/library/system.diagnostics.process(v=vs.110).aspx
             Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+
+            // Honor UI language customization
+            UILanguageOverride.Setup();
         }
 
         internal static bool TryGetBuiltInCommand(string commandName, out BuiltInCommandMetadata builtInCommand)

src/dotnet/UILanguageOverride.cs

@@ -0,0 +1,76 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.Globalization;
+
+namespace Microsoft.DotNet.Cli
+{
+    internal static class UILanguageOverride
+    {
+        private const string DOTNET_CLI_UI_LANGUAGE = nameof(DOTNET_CLI_UI_LANGUAGE);
+        private const string VSLANG = nameof(VSLANG);
+        private const string PreferredUILang = nameof(PreferredUILang);
+
+        public static void Setup()
+        {
+            CultureInfo language = GetOverriddenUILanguage();
+            if (language == null)
+            {
+                return;
+            }
+
+            // Make the current process respect the override.
+            CultureInfo.DefaultThreadCurrentUICulture = language;
+
+            // Pass down the override to other processes that we start via appropriate environment variables
+            // Do not override any environment variables that are already set as we do not want to clobber a more granular setting with our global setting.
+            SetIfNotAlreadySet(DOTNET_CLI_UI_LANGUAGE, language.Name);
+            SetIfNotAlreadySet(VSLANG, language.LCID); // for tools following VS guidelines to just work in CLI
+            SetIfNotAlreadySet(PreferredUILang, language.Name); // for C#/VB targets that pass $(PreferredUILang) to compiler
+        }
+
+        private static CultureInfo GetOverriddenUILanguage()
+        {
+            // DOTNET_CLI_UI_LANGUAGE=<culture name> is the main way for users to customize the CLI's UI language.
+            string dotnetCliLanguage = Environment.GetEnvironmentVariable(DOTNET_CLI_UI_LANGUAGE);
+            if (dotnetCliLanguage != null)
+            {
+                try
+                {
+                    return new CultureInfo(dotnetCliLanguage);
+                }
+                catch (CultureNotFoundException) { }
+            }
+
+            // VSLANG=<lcid> is set by VS and we respect that as well so that we will respect the VS 
+            // language preference if we're invoked by VS. 
+            string vsLang = Environment.GetEnvironmentVariable(VSLANG);
+            if (vsLang != null && int.TryParse(VSLANG, out int vsLcid))
+            {
+                try
+                {
+                    return new CultureInfo(vsLcid);
+                }
+                catch (ArgumentOutOfRangeException) { }
+                catch (CultureNotFoundException) { }
+            }
+
+            return null;
+        }
+
+        private static void SetIfNotAlreadySet(string environmentVariableName, string value)
+        {
+            string currentValue = Environment.GetEnvironmentVariable(environmentVariableName);
+            if (currentValue == null)
+            {
+                Environment.SetEnvironmentVariable(environmentVariableName, value);
+            }
+        }
+
+        private static void SetIfNotAlreadySet(string environmentVariableName, int value)
+        {
+            SetIfNotAlreadySet(environmentVariableName, value.ToString());
+        }
+    }
+}