diff --git a/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs b/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs
index 111a096a8e..7b618018a9 100644
--- a/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs
+++ b/src/Platform/Microsoft.Testing.Platform/Builder/TestApplication.cs
@@ -71,13 +71,17 @@ public static Task<ITestApplicationBuilder> CreateServerModeBuilderAsync(string[
     /// <returns>The task representing the asynchronous operation.</returns>
     public static async Task<ITestApplicationBuilder> CreateBuilderAsync(string[] args, TestApplicationOptions? testApplicationOptions = null)
     {
+        SystemEnvironment systemEnvironment = new();
+
+        // See AB#2304879.
+        UILanguageOverride.SetCultureSpecifiedByUser(systemEnvironment);
+
         // We get the time to save it in the logs for testcontrollers troubleshooting.
         SystemClock systemClock = new();
         DateTimeOffset createBuilderStart = systemClock.UtcNow;
         string createBuilderEntryTime = createBuilderStart.ToString("HH:mm:ss.fff", CultureInfo.InvariantCulture);
         testApplicationOptions ??= new TestApplicationOptions();
 
-        SystemEnvironment systemEnvironment = new();
         LaunchAttachDebugger(systemEnvironment);
 
         // First step is to parse the command line from where we get the second input layer.
diff --git a/src/Platform/Microsoft.Testing.Platform/Helpers/UILanguageOverride.cs b/src/Platform/Microsoft.Testing.Platform/Helpers/UILanguageOverride.cs
new file mode 100644
index 0000000000..e2244a0f58
--- /dev/null
+++ b/src/Platform/Microsoft.Testing.Platform/Helpers/UILanguageOverride.cs
@@ -0,0 +1,101 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.Globalization;
+
+using Microsoft.Testing.Platform.Helpers;
+
+namespace Microsoft.Testing.Platform;
+
+// Borrowed from dotnet/sdk with some tweaks to allow testing
+internal static class UILanguageOverride
+{
+#pragma warning disable SA1310 // Field names should not contain underscore - That's how we want to name the environment variables!
+    private const string TESTINGPLATFORM_UI_LANGUAGE = nameof(TESTINGPLATFORM_UI_LANGUAGE);
+    private const string DOTNET_CLI_UI_LANGUAGE = nameof(DOTNET_CLI_UI_LANGUAGE);
+#pragma warning restore SA1310 // Field names should not contain underscore
+
+    private const string VSLANG = nameof(VSLANG);
+    private const string PreferredUILang = nameof(PreferredUILang);
+
+    internal static void SetCultureSpecifiedByUser(IEnvironment environment)
+    {
+        CultureInfo? language = GetOverriddenUILanguage(environment);
+        if (language == null)
+        {
+            return;
+        }
+
+        ApplyOverrideToCurrentProcess(language);
+        FlowOverrideToChildProcesses(language, environment);
+    }
+
+    private static void ApplyOverrideToCurrentProcess(CultureInfo language)
+        => CultureInfo.DefaultThreadCurrentUICulture = language;
+
+    private static CultureInfo? GetOverriddenUILanguage(IEnvironment environment)
+    {
+        // For MTP, TESTINGPLATFORM_UI_LANGUAGE environment variable is the highest precedence.
+        string? testingPlatformLanguage = environment.GetEnvironmentVariable(TESTINGPLATFORM_UI_LANGUAGE);
+        if (testingPlatformLanguage is not null)
+        {
+            try
+            {
+                return CultureInfo.GetCultureInfo(testingPlatformLanguage);
+            }
+            catch (CultureNotFoundException)
+            {
+            }
+        }
+
+        // If TESTINGPLATFORM_UI_LANGUAGE is not set or is invalid, then 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 is not 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 FlowOverrideToChildProcesses(CultureInfo language, IEnvironment environment)
+    {
+        // 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(TESTINGPLATFORM_UI_LANGUAGE, language.Name, environment);
+        SetIfNotAlreadySet(DOTNET_CLI_UI_LANGUAGE, language.Name, environment);
+        SetIfNotAlreadySet(VSLANG, language.LCID.ToString(CultureInfo.CurrentCulture), environment); // for tools following VS guidelines to just work in CLI
+        SetIfNotAlreadySet(PreferredUILang, language.Name, environment); // for C#/VB targets that pass $(PreferredUILang) to compiler
+    }
+
+    private static void SetIfNotAlreadySet(string environmentVariableName, string value, IEnvironment environment)
+    {
+        string? currentValue = environment.GetEnvironmentVariable(environmentVariableName);
+        if (currentValue == null)
+        {
+            environment.SetEnvironmentVariable(environmentVariableName, value);
+        }
+    }
+}
diff --git a/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/RunsettingsTests.cs b/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/RunsettingsTests.cs
index 8b9ec37177..d7d87424a3 100644
--- a/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/RunsettingsTests.cs
+++ b/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/RunsettingsTests.cs
@@ -1,10 +1,12 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
+using System.Globalization;
 using System.Runtime.InteropServices;
 
 using Microsoft.Testing.Platform.Acceptance.IntegrationTests;
 using Microsoft.Testing.Platform.Acceptance.IntegrationTests.Helpers;
+using Microsoft.Testing.Platform.Helpers;
 
 namespace MSTest.Acceptance.IntegrationTests;
 
@@ -22,6 +24,27 @@ internal static IEnumerable<TestArgumentsEntry<string>> TfmList()
         yield return TargetFrameworks.NetFramework.First();
     }
 
+    internal static IEnumerable<TestArgumentsEntry<(string? TestingPlatformUILanguage, string? DotnetCLILanguage, string? VSLang, string ExpectedLocale)>> LocalizationTestCases()
+    {
+        // Show that TestingPlatformUILanguage is respected.
+        yield return new TestArgumentsEntry<(string?, string?, string?, string)>(("fr-FR", null, null, "fr-FR"), "TestingPlatformUILanguage: fr-FR, expected: fr-FR");
+
+        // Show that TestingPlatformUILanguage takes precedence over DotnetCLILanguage.
+        yield return new TestArgumentsEntry<(string?, string?, string?, string)>(("fr-FR", "it-IT", null, "fr-FR"), "TestingPlatformUILanguage: fr-FR, CLI: it-IT, expected: fr-FR");
+
+        // Show that DotnetCLILanguage is respected.
+        yield return new TestArgumentsEntry<(string?, string?, string?, string)>((null, "it-IT", null, "it-IT"), "CLI: it-IT, expected: it-IT");
+
+        // Show that DotnetCLILanguage takes precedence over VSLang.
+        yield return new TestArgumentsEntry<(string?, string?, string?, string)>((null, "it-IT", "fr-FR", "it-IT"), "CLI: it-IT, VSLang: fr-FR, expected: it-IT");
+
+        // Show that VSLang is respected.
+        yield return new TestArgumentsEntry<(string?, string?, string?, string)>((null, null, "it-IT", "it-IT"), "VSLang: it-IT, expected: it-IT");
+
+        // Show that TestingPlatformUILanguage takes precedence over everything.
+        yield return new TestArgumentsEntry<(string?, string?, string?, string)>(("fr-FR", "it-IT", "it-IT", "fr-FR"), "TestingPlatformUILanguage: fr-FR, CLI: it-IT, VSLang: it-IT, expected: fr-FR");
+    }
+
     [ArgumentsProvider(nameof(TfmList))]
     public async Task UnsupportedRunSettingsEntriesAreFlagged(string tfm)
     {
@@ -48,6 +71,35 @@ public async Task UnsupportedRunSettingsEntriesAreFlagged(string tfm)
         testHostResult.AssertOutputContains("Runsettings attribute 'TreatNoTestsAsError' is not supported by Microsoft.Testing.Platform and will be ignored");
     }
 
+    [ArgumentsProvider(nameof(LocalizationTestCases))]
+    public async Task UnsupportedRunSettingsEntriesAreFlagged_Localization((string? TestingPlatformUILanguage, string? DotnetCLILanguage, string? VSLang, string? ExpectedLocale) testArgument)
+    {
+        var testHost = TestHost.LocateFrom(_testAssetFixture.ProjectPath, TestAssetFixture.ProjectName, TargetFrameworks.NetCurrent.Arguments);
+        TestHostResult testHostResult = await testHost.ExecuteAsync("--settings my.runsettings", environmentVariables: new()
+        {
+            ["TESTINGPLATFORM_UI_LANGUAGE"] = testArgument.TestingPlatformUILanguage,
+            ["DOTNET_CLI_UI_LANGUAGE"] = testArgument.DotnetCLILanguage,
+            ["VSLANG"] = testArgument.VSLang is null ? null : new CultureInfo(testArgument.VSLang).LCID.ToString(CultureInfo.CurrentCulture),
+        });
+
+        // Assert
+        testHostResult.AssertExitCodeIs(0);
+
+        switch (testArgument.ExpectedLocale)
+        {
+            case "fr-FR":
+                testHostResult.AssertOutputContains("Les loggers Runsettings ne sont pas pris en charge par Microsoft.Testing.Platform et seront ignorés");
+                testHostResult.AssertOutputContains("Les datacollecteurs Runsettings ne sont pas pris en charge par Microsoft.Testing.Platform et seront ignorés");
+                break;
+            case "it-IT":
+                testHostResult.AssertOutputContains("I logger Runsettings non sono supportati da Microsoft.Testing.Platform e verranno ignorati");
+                testHostResult.AssertOutputContains("I datacollector Runsettings non sono supportati da Microsoft.Testing.Platform e verranno ignorati");
+                break;
+            default:
+                throw ApplicationStateGuard.Unreachable();
+        }
+    }
+
     [TestFixture(TestFixtureSharingStrategy.PerTestGroup)]
     public sealed class TestAssetFixture(AcceptanceFixture acceptanceFixture) : TestAssetFixtureBase(acceptanceFixture.NuGetGlobalPackagesFolder)
     {
diff --git a/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/ServerMode/ServerModeTestsBase.cs b/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/ServerMode/ServerModeTestsBase.cs
index 9cea463f82..d31bf62902 100644
--- a/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/ServerMode/ServerModeTestsBase.cs
+++ b/test/IntegrationTests/MSTest.Acceptance.IntegrationTests/ServerMode/ServerModeTestsBase.cs
@@ -16,7 +16,7 @@ namespace MSTest.Acceptance.IntegrationTests.Messages.V100;
 public partial /* for codegen regx */ class ServerModeTestsBase : AcceptanceTestBase
 {
     private static readonly string Root = RootFinder.Find();
-    private static readonly Dictionary<string, string> DefaultEnvironmentVariables = new()
+    private static readonly Dictionary<string, string?> DefaultEnvironmentVariables = new()
     {
         { "DOTNET_ROOT", $"{Root}/.dotnet" },
         { "DOTNET_INSTALL_DIR", $"{Root}/.dotnet" },
@@ -31,7 +31,7 @@ protected ServerModeTestsBase(ITestExecutionContext testExecutionContext)
 
     protected async Task<TestingPlatformClient> StartAsServerAndConnectToTheClientAsync(TestHost testHost)
     {
-        var environmentVariables = new Dictionary<string, string>(DefaultEnvironmentVariables);
+        var environmentVariables = new Dictionary<string, string?>(DefaultEnvironmentVariables);
         foreach (DictionaryEntry entry in Environment.GetEnvironmentVariables())
         {
             // Skip all unwanted environment variables.
@@ -80,7 +80,7 @@ protected async Task<TestingPlatformClient> StartAsServerAndConnectToTheClientAs
 
     protected async Task<TestingPlatformClient> StartAsServerAndConnectAsync(TestHost testHost, bool enableDiagnostic = false)
     {
-        var environmentVariables = new Dictionary<string, string>(DefaultEnvironmentVariables);
+        var environmentVariables = new Dictionary<string, string?>(DefaultEnvironmentVariables);
         foreach (DictionaryEntry entry in Environment.GetEnvironmentVariables())
         {
             // Skip all unwanted environment variables.
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CrashPlusHangDumpTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CrashPlusHangDumpTests.cs
index 9fc3d49b6e..44bd63c540 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CrashPlusHangDumpTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CrashPlusHangDumpTests.cs
@@ -20,7 +20,7 @@ public async Task CrashPlusHangDump_InCaseOfCrash_CreateCrashDump()
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "CrashPlusHangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 5m --crashdump --results-directory {resultDirectory}",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                         { "SLEEPTIMEMS1", "4000" },
                         { "SLEEPTIMEMS2", "600000" },
@@ -41,7 +41,7 @@ public async Task CrashPlusHangDump_InCaseOfHang_CreateHangDump()
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "CrashPlusHangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 8s --crashdump --results-directory {resultDirectory}",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                         { "SLEEPTIMEMS1", "4000" },
                         { "SLEEPTIMEMS2", "600000" },
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CustomBannerTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CustomBannerTests.cs
index ba88d37dae..4c3920dbfd 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CustomBannerTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/CustomBannerTests.cs
@@ -31,7 +31,7 @@ public async Task UsingNoBanner_InTheEnvironmentVars_TheBannerDoesNotAppear(stri
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "TESTINGPLATFORM_NOBANNER", "true" },
             });
@@ -46,7 +46,7 @@ public async Task UsingDotnetNoLogo_InTheEnvironmentVars_TheBannerDoesNotAppear(
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "DOTNET_NOLOGO", "true" },
             });
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/DiagnosticTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/DiagnosticTests.cs
index 3c20acdb5a..4a2fb47dab 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/DiagnosticTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/DiagnosticTests.cs
@@ -110,7 +110,7 @@ public async Task Diag_EnableWithEnvironmentVariables_Succeeded(string tfm)
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC, "1" },
             });
@@ -127,7 +127,7 @@ public async Task Diag_EnableWithEnvironmentVariables_Verbosity_Succeeded(string
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC, "1" },
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC_VERBOSITY, "Trace" },
@@ -145,7 +145,7 @@ public async Task Diag_EnableWithEnvironmentVariables_CustomPrefix_Succeeded(str
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC, "1" },
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC_OUTPUT_FILEPREFIX, "MyPrefix" },
@@ -163,7 +163,7 @@ public async Task Diag_EnableWithEnvironmentVariables_SynchronousWrite_Succeeded
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC, "1" },
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC_FILELOGGER_SYNCHRONOUSWRITE, "1" },
@@ -179,7 +179,7 @@ public async Task Diag_EnableWithEnvironmentVariables_Disable_Succeeded(string t
 
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             "--diagnostic",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_DIAGNOSTIC, "0" },
             });
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/ExitOnProcessExitTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/ExitOnProcessExitTests.cs
index d546f4e581..98b741b9e9 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/ExitOnProcessExitTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/ExitOnProcessExitTests.cs
@@ -21,7 +21,7 @@ public void ExitOnProcessExit_Succeed(string tfm)
 
         // Create the mutex name used to wait for the PID file created by the test host.
         string waitPid = Guid.NewGuid().ToString("N");
-        _ = testHost.ExecuteAsync(environmentVariables: new Dictionary<string, string> { { "WaitPid", waitPid } });
+        _ = testHost.ExecuteAsync(environmentVariables: new Dictionary<string, string?> { { "WaitPid", waitPid } });
 
         Process? process;
         var startTime = Stopwatch.StartNew();
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpOutputTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpOutputTests.cs
index e5bb4b6ff3..2309c7082e 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpOutputTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpOutputTests.cs
@@ -26,7 +26,7 @@ public async Task HangDump_Outputs_HangingTests_EvenWhenHangingTestsHaveTheSameD
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "HangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 8s --hangdump-type {format} --results-directory {resultDirectory} --no-progress",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "SLEEPTIMEMS1", "100" },
                 { "SLEEPTIMEMS2", "600000" },
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpTests.cs
index d524c88c57..4fe5ac589e 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/HangDumpTests.cs
@@ -21,7 +21,7 @@ public async Task HangDump_DefaultSetting_CreateDump(string tfm)
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "HangDump", tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 8s --results-directory {resultDirectory}",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                         { "SLEEPTIMEMS1", "4000" },
                         { "SLEEPTIMEMS2", "600000" },
@@ -37,7 +37,7 @@ public async Task HangDump_CustomFileName_CreateDump()
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "HangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 8s --hangdump-filename myhungdumpfile_%p.dmp --results-directory {resultDirectory}",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "SLEEPTIMEMS1", "4000" },
                 { "SLEEPTIMEMS2", "600000" },
@@ -55,7 +55,7 @@ public async Task HangDump_PathWithSpaces_CreateDump()
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "HangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"""--hangdump --hangdump-timeout 8s --hangdump-filename myhungdumpfile_%p.dmp --results-directory "{resultDirectory}" """,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "SLEEPTIMEMS1", "4000" },
                 { "SLEEPTIMEMS2", "20000" },
@@ -75,7 +75,7 @@ public async Task HangDump_Formats_CreateDump(string format)
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "HangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 8s --hangdump-type {format} --results-directory {resultDirectory}",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                         { "SLEEPTIMEMS1", "4000" },
                         { "SLEEPTIMEMS2", "600000" },
@@ -91,7 +91,7 @@ public async Task HangDump_InvalidFormat_ShouldFail()
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, "HangDump", TargetFrameworks.NetCurrent.Arguments);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             $"--hangdump --hangdump-timeout 8s --hangdump-type invalid --results-directory {resultDirectory}",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "SLEEPTIMEMS1", "4000" },
                 { "SLEEPTIMEMS2", "600000" },
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/IgnoreExitCodeTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/IgnoreExitCodeTests.cs
index 8739a515bb..b1b27a3e97 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/IgnoreExitCodeTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/IgnoreExitCodeTests.cs
@@ -113,7 +113,7 @@ public async Task If_IgnoreExitCode_Specified_Should_Return_Success_ExitCode(str
         var host = TestInfrastructure.TestHost.LocateFrom(assetPath, AssetName, tfm, buildConfiguration: buildConfiguration);
         TestHostResult hostResult = await host.ExecuteAsync(
             command: commandLine,
-            environmentVariables: new Dictionary<string, string>
+            environmentVariables: new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_EXITCODE_IGNORE, environmentVariable },
             });
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/MSBuildTests.Test.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/MSBuildTests.Test.cs
index 09b2e7129f..fa4a775ae4 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/MSBuildTests.Test.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/MSBuildTests.Test.cs
@@ -132,7 +132,7 @@ public async Task Invoke_DotnetTest_With_Arch_Switch_x86_Should_Work()
 
         string root = RootFinder.Find();
         string x86Muxer = Path.Combine(root, ".dotnet", "x86");
-        var dotnetRootX86 = new Dictionary<string, string>
+        var dotnetRootX86 = new Dictionary<string, string?>
         {
             { "DOTNET_ROOT_X86", x86Muxer },
         };
@@ -168,7 +168,7 @@ public async Task Invoke_DotnetTest_With_DOTNET_HOST_PATH_Should_Work()
 
         string root = RootFinder.Find();
         string dotnetHostPath = Path.Combine(root, ".dotnet", "dotnet.exe");
-        var dotnetHostPathEnvVar = new Dictionary<string, string>
+        var dotnetHostPathEnvVar = new Dictionary<string, string?>
         {
             { "DOTNET_HOST_PATH", dotnetHostPath },
         };
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/NoBannerTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/NoBannerTests.cs
index 83d2017756..599edb8d81 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/NoBannerTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/NoBannerTests.cs
@@ -32,7 +32,7 @@ public async Task UsingNoBanner_InTheEnvironmentVars_TheBannerDoesNotAppear(stri
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "TESTINGPLATFORM_NOBANNER", "true" },
             });
@@ -47,7 +47,7 @@ public async Task UsingDotnetNoLogo_InTheEnvironmentVars_TheBannerDoesNotAppear(
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             null,
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { "DOTNET_NOLOGO", "true" },
             });
diff --git a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/TelemetryTests.cs b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/TelemetryTests.cs
index c533062424..48cafde0c2 100644
--- a/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/TelemetryTests.cs
+++ b/test/IntegrationTests/Microsoft.Testing.Platform.Acceptance.IntegrationTests/TelemetryTests.cs
@@ -49,7 +49,7 @@ public async Task Telemetry_WhenOptingOutTelemetry_WithEnvironmentVariable_Telem
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             "--diagnostic",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.TESTINGPLATFORM_TELEMETRY_OPTOUT, "1" },
             },
@@ -76,7 +76,7 @@ public async Task Telemetry_WhenOptingOutTelemetry_With_DOTNET_CLI_EnvironmentVa
         var testHost = TestInfrastructure.TestHost.LocateFrom(_testAssetFixture.TargetAssetPath, AssetName, tfm);
         TestHostResult testHostResult = await testHost.ExecuteAsync(
             "--diagnostic",
-            new Dictionary<string, string>
+            new Dictionary<string, string?>
             {
                 { EnvironmentVariableConstants.DOTNET_CLI_TELEMETRY_OPTOUT, "1" },
             },
diff --git a/test/Utilities/Microsoft.Testing.TestInfrastructure/CommandLine.cs b/test/Utilities/Microsoft.Testing.TestInfrastructure/CommandLine.cs
index b535a6f366..73693a1896 100644
--- a/test/Utilities/Microsoft.Testing.TestInfrastructure/CommandLine.cs
+++ b/test/Utilities/Microsoft.Testing.TestInfrastructure/CommandLine.cs
@@ -41,7 +41,7 @@ public static int MaxOutstandingCommands
 
     public async Task RunAsync(
         string commandLine,
-        IDictionary<string, string>? environmentVariables = null)
+        IDictionary<string, string?>? environmentVariables = null)
     {
         int exitCode = await RunAsyncAndReturnExitCodeAsync(commandLine, environmentVariables);
         if (exitCode != 0)
@@ -57,7 +57,7 @@ public async Task RunAsync(
 
     public async Task<int> RunAsyncAndReturnExitCodeAsync(
         string commandLine,
-        IDictionary<string, string>? environmentVariables = null,
+        IDictionary<string, string?>? environmentVariables = null,
         string? workingDirectory = null,
         bool cleanDefaultEnvironmentVariableIfCustomAreProvided = false,
         int timeoutInSeconds = 60)
diff --git a/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetCli.cs b/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetCli.cs
index a786be904e..be08498905 100644
--- a/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetCli.cs
+++ b/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetCli.cs
@@ -52,7 +52,7 @@ public static async Task<DotnetMuxerResult> RunAsync(
         string args,
         string nugetGlobalPackagesFolder,
         string? workingDirectory = null,
-        Dictionary<string, string>? environmentVariables = null,
+        Dictionary<string, string?>? environmentVariables = null,
         bool failIfReturnValueIsNotZero = true,
         bool disableTelemetry = true,
         int timeoutInSeconds = 50,
@@ -64,7 +64,7 @@ public static async Task<DotnetMuxerResult> RunAsync(
         await s_maxOutstandingCommands_semaphore.WaitAsync();
         try
         {
-            environmentVariables ??= new Dictionary<string, string>();
+            environmentVariables ??= new Dictionary<string, string?>();
             foreach (DictionaryEntry entry in Environment.GetEnvironmentVariables())
             {
                 // Skip all unwanted environment variables.
@@ -123,7 +123,7 @@ public static async Task<DotnetMuxerResult> RunAsync(
         }
     }
 
-    private static async Task<DotnetMuxerResult> CallTheMuxerAsync(string args, Dictionary<string, string> environmentVariables, string? workingDirectory, int timeoutInSeconds, bool failIfReturnValueIsNotZero)
+    private static async Task<DotnetMuxerResult> CallTheMuxerAsync(string args, Dictionary<string, string?> environmentVariables, string? workingDirectory, int timeoutInSeconds, bool failIfReturnValueIsNotZero)
     {
         if (args.StartsWith("dotnet ", StringComparison.OrdinalIgnoreCase))
         {
diff --git a/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetMuxer.cs b/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetMuxer.cs
index af24e37218..cb798a263e 100644
--- a/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetMuxer.cs
+++ b/test/Utilities/Microsoft.Testing.TestInfrastructure/DotnetMuxer.cs
@@ -8,8 +8,8 @@ namespace Microsoft.Testing.TestInfrastructure;
 public class DotnetMuxer : IDisposable
 {
     private static readonly string Root = RootFinder.Find();
-    private static readonly IDictionary<string, string> DefaultEnvironmentVariables
-        = new Dictionary<string, string>
+    private static readonly IDictionary<string, string?> DefaultEnvironmentVariables
+        = new Dictionary<string, string?>
         {
                 { "DOTNET_ROOT", $"{Root}/.dotnet" },
                 { "DOTNET_INSTALL_DIR", $"{Root}/.dotnet" },
@@ -18,21 +18,21 @@ private static readonly IDictionary<string, string> DefaultEnvironmentVariables
         };
 
     private readonly string _dotnet;
-    private readonly IDictionary<string, string> _environmentVariables;
+    private readonly IDictionary<string, string?> _environmentVariables;
     private readonly CommandLine _commandLine;
     private bool _isDisposed;
 
     public DotnetMuxer()
         : this(
               DefaultEnvironmentVariables,
-              new Dictionary<string, string>(),
+              new Dictionary<string, string?>(),
               mergeDefaultEnvironmentVariables: true,
               useDefaultArtifactsPackages: true)
     {
     }
 
     public DotnetMuxer(
-        IDictionary<string, string> environmentVariables,
+        IDictionary<string, string?> environmentVariables,
         bool mergeEnvironmentVariables = true,
         bool useDefaultArtifactPackages = true)
         : this(
@@ -44,8 +44,8 @@ public DotnetMuxer(
     }
 
     private DotnetMuxer(
-        IDictionary<string, string> defaultEnvironmentVariables,
-        IDictionary<string, string> environmentVariables,
+        IDictionary<string, string?> defaultEnvironmentVariables,
+        IDictionary<string, string?> environmentVariables,
         bool mergeDefaultEnvironmentVariables = true,
         bool useDefaultArtifactsPackages = true)
     {
@@ -98,7 +98,7 @@ public async Task<int> ExecuteAsync(string arguments, string? workingDirectory =
     public async Task<int> ExecuteAsync(
         string arguments,
         string? workingDirectory,
-        IDictionary<string, string> environmentVariables,
+        IDictionary<string, string?> environmentVariables,
         int timeoutInSeconds = 60)
         => await _commandLine.RunAsyncAndReturnExitCodeAsync(
             $"{_dotnet} {arguments}",
@@ -107,22 +107,22 @@ public async Task<int> ExecuteAsync(
             cleanDefaultEnvironmentVariableIfCustomAreProvided: true,
             timeoutInSeconds: timeoutInSeconds);
 
-    private IDictionary<string, string> MergeEnvironmentVariables(
-        IDictionary<string, string> environmentVariables1,
-        IDictionary<string, string> environmentVariables2)
+    private IDictionary<string, string?> MergeEnvironmentVariables(
+        IDictionary<string, string?> environmentVariables1,
+        IDictionary<string, string?> environmentVariables2)
     {
         if (environmentVariables1.Count == 0)
         {
-            return new Dictionary<string, string>(environmentVariables2);
+            return new Dictionary<string, string?>(environmentVariables2);
         }
 
         if (environmentVariables2.Count == 0)
         {
-            return new Dictionary<string, string>(environmentVariables1);
+            return new Dictionary<string, string?>(environmentVariables1);
         }
 
-        IDictionary<string, string> mergedEnvironmentVariables = new Dictionary<string, string>(environmentVariables1);
-        foreach (KeyValuePair<string, string> kvp in environmentVariables2)
+        IDictionary<string, string?> mergedEnvironmentVariables = new Dictionary<string, string?>(environmentVariables1);
+        foreach (KeyValuePair<string, string?> kvp in environmentVariables2)
         {
             mergedEnvironmentVariables[kvp.Key] = kvp.Value;
         }
diff --git a/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessConfiguration.cs b/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessConfiguration.cs
index fc05c4bc25..783d50ffc9 100644
--- a/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessConfiguration.cs
+++ b/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessConfiguration.cs
@@ -13,7 +13,7 @@ public sealed class ProcessConfiguration
 
     public string? WorkingDirectory { get; init; }
 
-    public IDictionary<string, string>? EnvironmentVariables { get; init; }
+    public IDictionary<string, string?>? EnvironmentVariables { get; init; }
 
     public Action<IProcessHandle, string>? OnErrorOutput { get; init; }
 
diff --git a/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessFactory.cs b/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessFactory.cs
index 798e3fe9c0..4b8d554e4d 100644
--- a/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessFactory.cs
+++ b/test/Utilities/Microsoft.Testing.TestInfrastructure/ProcessFactory.cs
@@ -33,7 +33,7 @@ public static IProcessHandle Start(ProcessConfiguration config, bool cleanDefaul
                 processStartInfo.EnvironmentVariables.Clear();
             }
 
-            foreach (KeyValuePair<string, string> kvp in config.EnvironmentVariables)
+            foreach (KeyValuePair<string, string?> kvp in config.EnvironmentVariables)
             {
                 if (kvp.Value is null)
                 {
diff --git a/test/Utilities/Microsoft.Testing.TestInfrastructure/TestHost.cs b/test/Utilities/Microsoft.Testing.TestInfrastructure/TestHost.cs
index f6926cfaed..60676a556a 100644
--- a/test/Utilities/Microsoft.Testing.TestInfrastructure/TestHost.cs
+++ b/test/Utilities/Microsoft.Testing.TestInfrastructure/TestHost.cs
@@ -43,7 +43,7 @@ public static int MaxOutstandingExecutions
 
     public async Task<TestHostResult> ExecuteAsync(
         string? command = null,
-        Dictionary<string, string>? environmentVariables = null,
+        Dictionary<string, string?>? environmentVariables = null,
         bool disableTelemetry = true,
         int timeoutSeconds = 60)
     {
@@ -55,7 +55,7 @@ public async Task<TestHostResult> ExecuteAsync(
                 throw new InvalidOperationException($"Command should not start with module name '{_testHostModuleName}'.");
             }
 
-            environmentVariables ??= new Dictionary<string, string>();
+            environmentVariables ??= new Dictionary<string, string?>();
 
             if (disableTelemetry)
             {
@@ -71,7 +71,11 @@ public async Task<TestHostResult> ExecuteAsync(
                     continue;
                 }
 
-                environmentVariables.Add(key!, entry.Value!.ToString()!);
+                // We use TryAdd to let tests "overwrite" existing environment variables.
+                // Consider that the given dictionary has "TESTINGPLATFORM_UI_LANGUAGE" as a key.
+                // And also Environment.GetEnvironmentVariables() is returning TESTINGPLATFORM_UI_LANGUAGE.
+                // In that case, we do a "TryAdd" which effectively means the value from the original dictionary wins.
+                environmentVariables.TryAdd(key!, entry!.Value!.ToString()!);
             }
 
             // Define DOTNET_ROOT to point to the dotnet we install for this repository, to avoid