41 files changed, 237 insertions, 121 deletions

diff --git a/src/api/wix/WixToolset.Data/Localization.cs b/src/api/wix/WixToolset.Data/Localization.cs
index 70c096de..d7727f17 100644
--- a/src/api/wix/WixToolset.Data/Localization.cs
+++ b/src/api/wix/WixToolset.Data/Localization.cs
@@ -16,10 +16,19 @@ namespace WixToolset.Data
         private readonly Dictionary<string, LocalizedControl> localizedControls = new Dictionary<string, LocalizedControl>();
 
         /// <summary>
+        /// Instantiates a new localization object with default location.
+        /// </summary>
+        public Localization(int? codepage, int? summaryInformationCodepage, string culture, IDictionary<string, BindVariable> variables, IDictionary<string, LocalizedControl> localizedControls) : 
+            this(LocalizationLocation.Source, codepage, summaryInformationCodepage, culture, variables, localizedControls)
+        {
+        }
+
+        /// <summary>
         /// Instantiates a new localization object.
         /// </summary>
-        public Localization(int? codepage, int? summaryInformationCodepage, string culture, IDictionary<string, BindVariable> variables, IDictionary<string, LocalizedControl> localizedControls)
+        public Localization(LocalizationLocation location, int? codepage, int? summaryInformationCodepage, string culture, IDictionary<string, BindVariable> variables, IDictionary<string, LocalizedControl> localizedControls)
         {
+            this.Location = location;
             this.Codepage = codepage;
             this.SummaryInformationCodepage = summaryInformationCodepage;
             this.Culture = culture?.ToLowerInvariant() ?? String.Empty;
@@ -28,6 +37,11 @@ namespace WixToolset.Data
         }
 
         /// <summary>
+        /// Gets the location the localization came from.
+        /// </summary>
+        public LocalizationLocation Location { get; private set; }
+
+        /// <summary>
         /// Gets the codepage.
         /// </summary>
         /// <value>The codepage.</value>
@@ -57,9 +71,27 @@ namespace WixToolset.Data
         /// <value>The localized controls.</value>
         public ICollection<KeyValuePair<string, LocalizedControl>> LocalizedControls => this.localizedControls;
 
+        /// <summary>
+        /// Updates the location, if the location is a higher state than the current state.
+        /// </summary>
+        /// <param name="location">Location to update to.</param>
+        /// <returns>This localization object.</returns>
+        public Localization UpdateLocation(LocalizationLocation location)
+        {
+            if (this.Location < location)
+            {
+                this.Location = location;
+            }
+
+            return this;
+        }
+
         internal JsonObject Serialize()
         {
-            var jsonObject = new JsonObject();
+            var jsonObject = new JsonObject()
+            {
+                { "location", this.Location.ToString().ToLowerInvariant() }
+            };
 
             if (this.Codepage.HasValue)
             {
@@ -108,6 +140,7 @@ namespace WixToolset.Data
 
         internal static Localization Deserialize(JsonObject jsonObject)
         {
+            var location = jsonObject.GetEnumOrDefault("location", LocalizationLocation.Source);
             var codepage = jsonObject.GetValueOrDefault("codepage", null);
             var summaryCodepage = jsonObject.GetValueOrDefault("summaryCodepage", null);
             var culture = jsonObject.GetValueOrDefault<string>("culture");
@@ -131,7 +164,7 @@ namespace WixToolset.Data
                 }
             }
 
-            return new Localization(codepage, summaryCodepage, culture, variables, controls);
+            return new Localization(location, codepage, summaryCodepage, culture, variables, controls);
         }
     }
 }
diff --git a/src/api/wix/WixToolset.Data/LocalizationLocation.cs b/src/api/wix/WixToolset.Data/LocalizationLocation.cs
new file mode 100644
index 00000000..773c4d9f
--- /dev/null
+++ b/src/api/wix/WixToolset.Data/LocalizationLocation.cs
@@ -0,0 +1,31 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information.
+
+namespace WixToolset.Data
+{
+    /// <summary>
+    /// Location where the localization was loaded.
+    /// </summary>
+    public enum LocalizationLocation
+    {
+        /// <summary>
+        /// Localization loaded from .wxl source file.
+        /// </summary>
+        Source,
+
+        /// <summary>
+        /// Localization loaded from .wixlib library.
+        /// </summary>
+        Library,
+
+        /// <summary>
+        /// Localization loaded from .wixlib library within .wixext WixExtension.
+        /// </summary>
+        Extension,
+
+        /// <summary>
+        /// Localization placed in .wixext WixExtension as the default culture localization for the
+        /// WixExtension.
+        /// </summary>
+        ExtensionDefaultCulture
+    }
+}
diff --git a/src/api/wix/WixToolset.Data/LocalizedControl.cs b/src/api/wix/WixToolset.Data/LocalizedControl.cs
index 1252842b..9f59ce46 100644
--- a/src/api/wix/WixToolset.Data/LocalizedControl.cs
+++ b/src/api/wix/WixToolset.Data/LocalizedControl.cs
@@ -45,7 +45,10 @@ namespace WixToolset.Data
         /// Get key for a localized control.
         /// </summary>
         /// <returns>The localized control id.</returns>
-        public string GetKey() => LocalizedControl.GetKey(this.Dialog, this.Control);
+        public string GetKey()
+        {
+            return LocalizedControl.GetKey(this.Dialog, this.Control);
+        }
 
         /// <summary>
         /// Get key for a localized control.
@@ -53,7 +56,10 @@ namespace WixToolset.Data
         /// <param name="dialog">The optional id of the control's dialog.</param>
         /// <param name="control">The id of the control.</param>
         /// <returns>The localized control id.</returns>
-        public static string GetKey(string dialog, string control) => String.Concat(dialog, "/", control);
+        public static string GetKey(string dialog, string control)
+        {
+            return String.Concat(dialog, "/", control);
+        }
 
         internal JsonObject Serialize()
         {
diff --git a/src/api/wix/WixToolset.Extensibility/BaseExtensionData.cs b/src/api/wix/WixToolset.Extensibility/BaseExtensionData.cs
index e4a10fd9..291b1014 100644
--- a/src/api/wix/WixToolset.Extensibility/BaseExtensionData.cs
+++ b/src/api/wix/WixToolset.Extensibility/BaseExtensionData.cs
@@ -2,6 +2,7 @@
 
 namespace WixToolset.Extensibility
 {
+    using System;
     using WixToolset.Data;
 
     /// <summary>
@@ -10,8 +11,9 @@ namespace WixToolset.Extensibility
     public abstract class BaseExtensionData : IExtensionData
     {
         /// <summary>
-        /// See <see cref="IExtensionData.DefaultCulture"/>
+        /// Obsolete in WiX v5. Use the WixLocalization/@ExtensionDefaultCulture attribute in the wxl file instead.
         /// </summary>
+        [Obsolete("Set the ExtensionDefaultCulture attribute in the WixLocalization source file instead.")]
         public virtual string DefaultCulture => null;
 
         /// <summary>
diff --git a/src/api/wix/WixToolset.Extensibility/IExtensionData.cs b/src/api/wix/WixToolset.Extensibility/IExtensionData.cs
index 823e2beb..e6565f4a 100644
--- a/src/api/wix/WixToolset.Extensibility/IExtensionData.cs
+++ b/src/api/wix/WixToolset.Extensibility/IExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.Extensibility
     public interface IExtensionData
     {
         /// <summary>
-        /// Gets the optional default culture.
-        /// </summary>
-        /// <value>The optional default culture.</value>
-        string DefaultCulture { get; }
-
-        /// <summary>
         ///
         /// </summary>
         /// <param name="name"></param>
diff --git a/src/ext/Bal/wixext/BalExtensionData.cs b/src/ext/Bal/wixext/BalExtensionData.cs
index 55daf005..9dc99705 100644
--- a/src/ext/Bal/wixext/BalExtensionData.cs
+++ b/src/ext/Bal/wixext/BalExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.Bal
     /// </summary>
     public sealed class BalExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = BalSymbolDefinitions.ByName(name);
diff --git a/src/ext/ComPlus/wixext/ComPlusExtensionData.cs b/src/ext/ComPlus/wixext/ComPlusExtensionData.cs
index 9cd5341e..5e9d225d 100644
--- a/src/ext/ComPlus/wixext/ComPlusExtensionData.cs
+++ b/src/ext/ComPlus/wixext/ComPlusExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.ComPlus
     /// </summary>
     public sealed class ComPlusExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = ComPlusSymbolDefinitions.ByName(name);
diff --git a/src/ext/ComPlus/wixlib/en-us.wxl b/src/ext/ComPlus/wixlib/en-us.wxl
index 0ec0f55c..592cc641 100644
--- a/src/ext/ComPlus/wixlib/en-us.wxl
+++ b/src/ext/ComPlus/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
   <String Id="msierrComPlusCannotConnect" Overridable="yes" Value="Cannot connect to the COM+ admin catalog.  ([2]   [3]   [4]   [5])" />
   <String Id="msierrComPlusPartitionReadFailed" Overridable="yes" Value="Failed to read COM+ partitions.  ([2]   [3]   [4]   [5])" />
   <String Id="msierrComPlusPartitionRoleReadFailed" Overridable="yes" Value="Failed to read COM+ partition roles.  ([2]   [3]   [4]   [5])" />
diff --git a/src/ext/Dependency/wixext/DependencyExtensionData.cs b/src/ext/Dependency/wixext/DependencyExtensionData.cs
index 2f30c2bf..c3c8fc03 100644
--- a/src/ext/Dependency/wixext/DependencyExtensionData.cs
+++ b/src/ext/Dependency/wixext/DependencyExtensionData.cs
@@ -11,12 +11,6 @@ namespace WixToolset.Dependency
     public sealed class DependencyExtensionData : BaseExtensionData
     {
         /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
-        /// <summary>
         /// Gets the contained .wixlib content.
         /// </summary>
         /// <param name="symbolDefinitions">Strong typed symbold definitions.</param>
diff --git a/src/ext/Dependency/wixlib/en-us.wxl b/src/ext/Dependency/wixlib/en-us.wxl
index ca0a3ea6..9d6cc63a 100644
--- a/src/ext/Dependency/wixlib/en-us.wxl
+++ b/src/ext/Dependency/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-US" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-US" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <String Id="msierrDependencyMissingDependencies" Overridable="yes" Value="If you continue with this install, the product may not work properly because [2] or more dependencies are missing. Do you want to continue with this install anyway?" />
     <String Id="msierrDependencyHasDependents" Overridable="yes" Value="If you continue with this uninstall, [2] or more products may stop working properly. Do you want to continue with this uninstall anyway?" />
 </WixLocalization>
diff --git a/src/ext/DirectX/wixext/DirectXExtensionData.cs b/src/ext/DirectX/wixext/DirectXExtensionData.cs
index d61af23f..e0d81979 100644
--- a/src/ext/DirectX/wixext/DirectXExtensionData.cs
+++ b/src/ext/DirectX/wixext/DirectXExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.DirectX
     /// </summary>
     public sealed class DirectXExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override Intermediate GetLibrary(ISymbolDefinitionCreator symbolDefinitions)
         {
             return Intermediate.Load(typeof(DirectXExtensionData).Assembly, "WixToolset.DirectX.directx.wixlib", symbolDefinitions);
diff --git a/src/ext/Firewall/wixext/FirewallExtensionData.cs b/src/ext/Firewall/wixext/FirewallExtensionData.cs
index 7481d993..55a7b999 100644
--- a/src/ext/Firewall/wixext/FirewallExtensionData.cs
+++ b/src/ext/Firewall/wixext/FirewallExtensionData.cs
@@ -7,8 +7,6 @@ namespace WixToolset.Firewall
 
     public sealed class FirewallExtensionData : BaseExtensionData
     {
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = FirewallSymbolDefinitions.ByName(name);
diff --git a/src/ext/Firewall/wixlib/en-us.wxl b/src/ext/Firewall/wixlib/en-us.wxl
index 52dbab55..64c5a1ac 100644
--- a/src/ext/Firewall/wixlib/en-us.wxl
+++ b/src/ext/Firewall/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <String Id="msierrFirewallCannotConnect" Overridable="yes" Value="Cannot connect to Windows Firewall.  ([2]   [3]   [4]   [5])" />
     
     <String Id="WixSchedFirewallExceptionsInstall" Overridable="yes" Value="Configuring Windows Firewall" />
diff --git a/src/ext/Http/wixext/HttpExtensionData.cs b/src/ext/Http/wixext/HttpExtensionData.cs
index 04e3dcee..ff6934bd 100644
--- a/src/ext/Http/wixext/HttpExtensionData.cs
+++ b/src/ext/Http/wixext/HttpExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.Http
     /// </summary>
     public sealed class HttpExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = HttpSymbolDefinitions.ByName(name);
diff --git a/src/ext/Http/wixlib/en-us.wxl b/src/ext/Http/wixlib/en-us.wxl
index de3e93cd..44731a5f 100644
--- a/src/ext/Http/wixlib/en-us.wxl
+++ b/src/ext/Http/wixlib/en-us.wxl
@@ -1,6 +1,6 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <String Id="WixSchedHttpUrlReservationsInstall" Overridable="yes" Value="Preparing to configure Windows HTTP Server" />
     <String Id="WixSchedHttpUrlReservationsUninstall" Overridable="yes" Value="Preparing to configure Windows HTTP Server" />
     <String Id="WixRollbackHttpUrlReservationsInstall" Overridable="yes" Value="Rolling back Windows HTTP Server configuration" />
diff --git a/src/ext/Iis/wixext/IIsExtensionData.cs b/src/ext/Iis/wixext/IIsExtensionData.cs
index 6a0e1f09..ccbe362d 100644
--- a/src/ext/Iis/wixext/IIsExtensionData.cs
+++ b/src/ext/Iis/wixext/IIsExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.Iis
     /// </summary>
     public sealed class IIsExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = IisSymbolDefinitions.ByName(name);
diff --git a/src/ext/Iis/wixlib/en-us.wxl b/src/ext/Iis/wixlib/en-us.wxl
index 1b6096fb..65bc1435 100644
--- a/src/ext/Iis/wixlib/en-us.wxl
+++ b/src/ext/Iis/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <String Id="msierrIISCannotConnect" Overridable="yes" Value="Cannot connect to Internet Information Server.  ([2]   [3]   [4]   [5])" />
     <String Id="msierrIISFailedReadWebSite" Overridable="yes" Value="Failed while processing WebSites.   ([2]   [3]   [4]   [5])" />
     <String Id="msierrIISFailedReadWebDirs" Overridable="yes" Value="Failed while processing WebDirs.  ([2]   [3]   [4]   [5])" />
diff --git a/src/ext/Msmq/wixext/MsmqExtensionData.cs b/src/ext/Msmq/wixext/MsmqExtensionData.cs
index 91485724..f434001f 100644
--- a/src/ext/Msmq/wixext/MsmqExtensionData.cs
+++ b/src/ext/Msmq/wixext/MsmqExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.Msmq
     /// </summary>
     public sealed class MsmqExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = MsmqSymbolDefinitions.ByName(name);
diff --git a/src/ext/Msmq/wixlib/en-us.wxl b/src/ext/Msmq/wixlib/en-us.wxl
index 3252fc17..9965ca51 100644
--- a/src/ext/Msmq/wixlib/en-us.wxl
+++ b/src/ext/Msmq/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
   <String Id="MessageQueuingExecuteInstall" Overridable="yes" Value="Configuring message queues" />
   <String Id="MessageQueuingExecuteInstallTemplate" Overridable="yes" Value="Queue: [1]" />
   <String Id="MessageQueuingExecuteUninstall" Overridable="yes" Value="Configuring message queues" />
diff --git a/src/ext/NetFx/wixext/NetFxExtensionData.cs b/src/ext/NetFx/wixext/NetFxExtensionData.cs
index 8dd9e003..61d618cf 100644
--- a/src/ext/NetFx/wixext/NetFxExtensionData.cs
+++ b/src/ext/NetFx/wixext/NetFxExtensionData.cs
@@ -10,8 +10,6 @@ namespace WixToolset.Netfx
     /// </summary>
     public sealed class NetfxExtensionData : BaseExtensionData
     {
-        public override string DefaultCulture => "en-US";
-        
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = NetfxSymbolDefinitions.ByName(name);
diff --git a/src/ext/PowerShell/wixext/PSExtensionData.cs b/src/ext/PowerShell/wixext/PSExtensionData.cs
index 66627942..8ba83498 100644
--- a/src/ext/PowerShell/wixext/PSExtensionData.cs
+++ b/src/ext/PowerShell/wixext/PSExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.PowerShell
     /// </summary>
     public sealed class PSExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override Intermediate GetLibrary(ISymbolDefinitionCreator symbolDefinitions)
         {
             return Intermediate.Load(typeof(PSExtensionData).Assembly, "WixToolset.PowerShell.powershell.wixlib", symbolDefinitions);
diff --git a/src/ext/Sql/wixext/SqlExtensionData.cs b/src/ext/Sql/wixext/SqlExtensionData.cs
index 60de94fe..77a86bde 100644
--- a/src/ext/Sql/wixext/SqlExtensionData.cs
+++ b/src/ext/Sql/wixext/SqlExtensionData.cs
@@ -10,12 +10,6 @@ namespace WixToolset.Sql
     /// </summary>
     public sealed class SqlExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = SqlSymbolDefinitions.ByName(name);
diff --git a/src/ext/Sql/wixlib/en-us.wxl b/src/ext/Sql/wixlib/en-us.wxl
index 9e6e8765..654970a8 100644
--- a/src/ext/Sql/wixlib/en-us.wxl
+++ b/src/ext/Sql/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <String Id="msierrSQLFailedCreateDatabase" Overridable="yes" Value="Error [2]: failed to create SQL database: [3], error detail: [4]." />
     <String Id="msierrSQLFailedDropDatabase" Overridable="yes" Value="Error [2]: failed to drop SQL database: [3], error detail: [4]." />
     <String Id="msierrSQLFailedConnectDatabase" Overridable="yes" Value="Failed to connect to SQL database.  ([2]   [3]   [4]   [5])" />
diff --git a/src/ext/UI/wixext/UIExtensionData.cs b/src/ext/UI/wixext/UIExtensionData.cs
index 32557029..349a5912 100644
--- a/src/ext/UI/wixext/UIExtensionData.cs
+++ b/src/ext/UI/wixext/UIExtensionData.cs
@@ -7,8 +7,6 @@ namespace WixToolset.UI
 
     public sealed class UIExtensionData : BaseExtensionData
     {
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = null;
diff --git a/src/ext/UI/wixlib/WixUI_en-us.wxl b/src/ext/UI/wixlib/WixUI_en-us.wxl
index c9904b61..a06f8b17 100644
--- a/src/ext/UI/wixlib/WixUI_en-us.wxl
+++ b/src/ext/UI/wixlib/WixUI_en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-US" Codepage="1252" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-US" ExtensionDefaultCulture="yes" Codepage="1252" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <!-- _locID@Culture="en-US" _locComment="American English" -->
     <!-- _locID@Codepage="1252" _locComment="Windows-1252" -->
 
diff --git a/src/ext/Util/wixext/UtilExtensionData.cs b/src/ext/Util/wixext/UtilExtensionData.cs
index d3ca3358..c9de2876 100644
--- a/src/ext/Util/wixext/UtilExtensionData.cs
+++ b/src/ext/Util/wixext/UtilExtensionData.cs
@@ -7,8 +7,6 @@ namespace WixToolset.Util
 
     public sealed class UtilExtensionData : BaseExtensionData
     {
-        public override string DefaultCulture => "en-US";
-
         public override bool TryGetSymbolDefinitionByName(string name, out IntermediateSymbolDefinition symbolDefinition)
         {
             symbolDefinition = UtilSymbolDefinitions.ByName(name);
diff --git a/src/ext/Util/wixlib/en-us.wxl b/src/ext/Util/wixlib/en-us.wxl
index fc2db184..b144989e 100644
--- a/src/ext/Util/wixlib/en-us.wxl
+++ b/src/ext/Util/wixlib/en-us.wxl
@@ -1,7 +1,7 @@
 <!-- Copyright (c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information. -->
 
 
-<WixLocalization Culture="en-us" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
     <String Id="msierrUSRFailedUserCreate" Overridable="yes" Value="Failed to create user.  ([2]   [3]   [4]   [5])" />
     <String Id="msierrUSRFailedUserCreatePswd" Overridable="yes" Value="Failed to create user due to invalid password.  ([2]   [3]   [4]   [5])" />
     <String Id="msierrUSRFailedUserGroupAdd" Overridable="yes" Value="Failed to add user to group.  ([2]   [3]   [4]   [5])" />
diff --git a/src/ext/VisualStudio/wixext/VSExtensionData.cs b/src/ext/VisualStudio/wixext/VSExtensionData.cs
index 82c5b118..9477ce67 100644
--- a/src/ext/VisualStudio/wixext/VSExtensionData.cs
+++ b/src/ext/VisualStudio/wixext/VSExtensionData.cs
@@ -7,12 +7,6 @@ namespace WixToolset.VisualStudio
 
     public sealed class VSExtensionData : BaseExtensionData
     {
-        /// <summary>
-        /// Gets the default culture.
-        /// </summary>
-        /// <value>The default culture.</value>
-        public override string DefaultCulture => "en-US";
-
         public override Intermediate GetLibrary(ISymbolDefinitionCreator symbolDefinitions)
         {
             return Intermediate.Load(typeof(VSExtensionData).Assembly, "WixToolset.VisualStudio.vs.wixlib", symbolDefinitions);
diff --git a/src/wix/WixToolset.Core/Librarian.cs b/src/wix/WixToolset.Core/Librarian.cs
index f6357c6c..8f5239c0 100644
--- a/src/wix/WixToolset.Core/Librarian.cs
+++ b/src/wix/WixToolset.Core/Librarian.cs
@@ -63,6 +63,11 @@ namespace WixToolset.Core
                     return null;
                 }
 
+                foreach (var localization in localizationsByCulture.Values)
+                {
+                    localization.UpdateLocation(LocalizationLocation.Library);
+                }
+
                 trackedFiles = this.ResolveFilePathsToEmbed(context, sections);
 
                 if (this.Messaging.EncounteredError)
diff --git a/src/wix/WixToolset.Core/Linker.cs b/src/wix/WixToolset.Core/Linker.cs
index 5c021ca0..59728741 100644
--- a/src/wix/WixToolset.Core/Linker.cs
+++ b/src/wix/WixToolset.Core/Linker.cs
@@ -87,6 +87,14 @@ namespace WixToolset.Core
                     if (library != null)
                     {
                         sections.AddRange(library.Sections);
+
+                        if (library.Localizations?.Count > 0)
+                        {
+                            // Include localizations from the extension data and be sure to note that the localization came from
+                            // an extension. It is important to remember whiche localization came from an extension when filtering
+                            // localizations during the resolve process later.
+                            localizations.AddRange(library.Localizations.Select(l => l.UpdateLocation(LocalizationLocation.Extension)));
+                        }
                     }
                 }
 
diff --git a/src/wix/WixToolset.Core/LocalizationParser.cs b/src/wix/WixToolset.Core/LocalizationParser.cs
index 995efe40..29d3d932 100644
--- a/src/wix/WixToolset.Core/LocalizationParser.cs
+++ b/src/wix/WixToolset.Core/LocalizationParser.cs
@@ -86,6 +86,7 @@ namespace WixToolset.Core
         private static Localization ParseWixLocalizationElement(IMessaging messaging, XElement node)
         {
             var sourceLineNumbers = SourceLineNumber.CreateFromXObject(node);
+            var location = LocalizationLocation.Source;
             int? codepage = null;
             int? summaryInformationCodepage = null;
             string culture = null;
@@ -96,6 +97,13 @@ namespace WixToolset.Core
                 {
                     switch (attrib.Name.LocalName)
                     {
+                        case "ExtensionDefaultCulture":
+                            if (Common.GetAttributeYesNoValue(messaging, sourceLineNumbers, attrib) == YesNoType.Yes)
+                            {
+                                location = LocalizationLocation.ExtensionDefaultCulture;
+                            }
+                            break;
+
                         case "Codepage":
                             codepage = Common.GetValidCodePage(attrib.Value, allowNoChange: true, onlyAnsi: false, sourceLineNumbers);
                             break;
@@ -147,7 +155,7 @@ namespace WixToolset.Core
                 }
             }
 
-            return messaging.EncounteredError ? null : new Localization(codepage, summaryInformationCodepage, culture, variables, localizedControls);
+            return messaging.EncounteredError ? null : new Localization(location, codepage, summaryInformationCodepage, culture, variables, localizedControls);
         }
 
         /// <summary>
diff --git a/src/wix/WixToolset.Core/Resolver.cs b/src/wix/WixToolset.Core/Resolver.cs
index 2eebc80a..97f0223f 100644
--- a/src/wix/WixToolset.Core/Resolver.cs
+++ b/src/wix/WixToolset.Core/Resolver.cs
@@ -230,21 +230,6 @@ namespace WixToolset.Core
 
             AddFilteredLocalizations(result, filter, localizations);
 
-            // Filter localizations provided by extensions with data.
-            var creator = context.ServiceProvider.GetService<ISymbolDefinitionCreator>();
-
-            foreach (var data in context.ExtensionData)
-            {
-                var library = data.GetLibrary(creator);
-
-                if (library?.Localizations != null && library.Localizations.Any())
-                {
-                    var extensionFilter = (!filter.Any() && data.DefaultCulture != null) ? new[] { data.DefaultCulture } : filter;
-
-                    AddFilteredLocalizations(result, extensionFilter, library.Localizations);
-                }
-            }
-
             return result;
         }
 
@@ -265,10 +250,15 @@ namespace WixToolset.Core
 
         private static void AddFilteredLocalizations(List<Localization> result, IEnumerable<string> filter, IEnumerable<Localization> localizations)
         {
-            // If there is no filter, return all localizations.
+            // If there is no filter, return all localizations provided by the user (either as a .wxl or from a .wixlib on the command-line)
+            // **and only** the extension's default culture localizations.
             if (!filter.Any())
             {
-                result.AddRange(localizations);
+                // The filter turns out to be really simple, skip localizations that came from an extension (LocalizationLocation.Extension)
+                // but keep those that are marked as the extension's default culture (LocalizationLocation.ExtensionDefaultCulture).
+                var filtered = localizations.Where(l => l.Location != LocalizationLocation.Extension);
+
+                result.AddRange(filtered);
             }
             else // filter localizations in order specified by the filter
             {
diff --git a/src/wix/test/CompileCoreTestExtensionWixlib/CompileCoreTestExtensionWixlib.csproj b/src/wix/test/CompileCoreTestExtensionWixlib/CompileCoreTestExtensionWixlib.csproj
index b526a209..2938be48 100644
--- a/src/wix/test/CompileCoreTestExtensionWixlib/CompileCoreTestExtensionWixlib.csproj
+++ b/src/wix/test/CompileCoreTestExtensionWixlib/CompileCoreTestExtensionWixlib.csproj
@@ -8,6 +8,8 @@
     <OutputType>Exe</OutputType>
     <SignOutput>false</SignOutput>
     <RollForward>Major</RollForward>
+
+    <WixlibPath>$(BaseOutputPath)TestData\$(Configuration)\example.wixlib</WixlibPath>
   </PropertyGroup>
 
   <ItemGroup>
@@ -15,20 +17,19 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ExtensionWxs Include="..\Example.Extension\Data\example.wxs">
-      <WixlibPath>$(BaseOutputPath)TestData\$(Configuration)\example.wixlib</WixlibPath>
-    </ExtensionWxs>
+    <ExtensionSource Include="..\Example.Extension\Data\example.wxs" />
+    <ExtensionSource Include="..\Example.Extension\Data\example.en-us.wxl" />
+    <ExtensionSource Include="..\Example.Extension\Data\example.ja-jp.wxl" />
   </ItemGroup>
 
   <Target Name="BuildExtensionWixlibs"
           AfterTargets="AfterBuild"
-          Inputs="@(ExtensionWxs)"
-          Outputs="%(ExtensionWxs.WixlibPath)"
+          Inputs="@(ExtensionSource)"
+          Outputs="$(WixlibPath)"
           Condition=" '$(NCrunch)'!='1' ">
 
-    <Exec Command="dotnet @(TargetPathWithTargetPlatformMoniker) &quot;$(IntermediateOutputPath) &quot; &quot;%(ExtensionWxs.WixlibPath)&quot; &quot;%(ExtensionWxs.Filename)%(ExtensionWxs.Extension)&quot;"
-          WorkingDirectory="%(ExtensionWxs.RelativeDir)" />
+    <Exec Command="dotnet @(TargetPathWithTargetPlatformMoniker) &quot;$(IntermediateOutputPath) &quot; &quot;$(WixlibPath)&quot; &quot;@(ExtensionSource)&quot;" />
 
-    <Message Importance="high" Text="@(ExtensionWxs) -&gt; %(ExtensionWxs.WixlibPath)" />
+    <Message Importance="high" Text="@(ExtensionSource) -&gt; $(WixlibPath)" />
   </Target>
 </Project>
diff --git a/src/wix/test/CompileCoreTestExtensionWixlib/Program.cs b/src/wix/test/CompileCoreTestExtensionWixlib/Program.cs
index a6c88ddb..bb65b467 100644
--- a/src/wix/test/CompileCoreTestExtensionWixlib/Program.cs
+++ b/src/wix/test/CompileCoreTestExtensionWixlib/Program.cs
@@ -1,6 +1,7 @@
 // Copyright(c) .NET Foundation and contributors. All rights reserved. Licensed under the Microsoft Reciprocal License. See LICENSE.TXT file in the project root for full license information.
 
 using System.Collections.Generic;
+using System.IO;
 using WixInternal.Core.TestPackage;
 
 namespace CompileCoreTestExtensionWixlib
@@ -17,8 +18,6 @@ namespace CompileCoreTestExtensionWixlib
             var buildArgs = new List<string>();
             buildArgs.Add("build");
             buildArgs.Add("-bindfiles");
-            buildArgs.Add("-bindpath");
-            buildArgs.Add("Data");
             buildArgs.Add("-intermediateFolder");
             buildArgs.Add(intermediateFolder);
             buildArgs.Add("-o");
@@ -26,6 +25,11 @@ namespace CompileCoreTestExtensionWixlib
 
             foreach (var path in args[2].Split(';'))
             {
+                var folder = Path.GetDirectoryName(Path.GetFullPath(path));
+
+                buildArgs.Add("-bindpath");
+                buildArgs.Add(folder);
+
                 buildArgs.Add(path);
             }
 
diff --git a/src/wix/test/Example.Extension/Data/example.en-us.wxl b/src/wix/test/Example.Extension/Data/example.en-us.wxl
new file mode 100644
index 00000000..8dfc1c30
--- /dev/null
+++ b/src/wix/test/Example.Extension/Data/example.en-us.wxl
@@ -0,0 +1,3 @@
+<WixLocalization Culture="en-us" ExtensionDefaultCulture="yes" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+  <String Id="ExampleString" Overridable="yes" Value="en-us" />
+</WixLocalization>
diff --git a/src/wix/test/Example.Extension/Data/example.ja-jp.wxl b/src/wix/test/Example.Extension/Data/example.ja-jp.wxl
new file mode 100644
index 00000000..32d52e83
--- /dev/null
+++ b/src/wix/test/Example.Extension/Data/example.ja-jp.wxl
@@ -0,0 +1,3 @@
+<WixLocalization Culture="ja-jp" xmlns="http://wixtoolset.org/schemas/v4/wxl">
+  <String Id="ExampleString" Overridable="yes" Value="ja-jp" />
+</WixLocalization>
diff --git a/src/wix/test/Example.Extension/Data/example.wxs b/src/wix/test/Example.Extension/Data/example.wxs
index af5d5086..a5298ac3 100644
--- a/src/wix/test/Example.Extension/Data/example.wxs
+++ b/src/wix/test/Example.Extension/Data/example.wxs
@@ -4,11 +4,17 @@
 
     <Binary Id="BinFromWir" SourceFile="example.txt" />
   </Fragment>
+
+  <Fragment>
+    <Property Id="LocalizedProperty" Value="!(loc.ExampleString)" />
+  </Fragment>
+
   <Fragment>
     <BootstrapperApplication Id="fakeba">
       <BootstrapperApplicationDll SourceFile="example.txt" />
     </BootstrapperApplication>
   </Fragment>
+
   <Fragment>
     <BundleExtension Id="ExampleBundleExtension" SourceFile="example.txt" />
   </Fragment>
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/ExtensionFixture.cs b/src/wix/test/WixToolsetTest.CoreIntegration/ExtensionFixture.cs
index 429226f5..a59187b6 100644
--- a/src/wix/test/WixToolsetTest.CoreIntegration/ExtensionFixture.cs
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/ExtensionFixture.cs
@@ -18,7 +18,7 @@ namespace WixToolsetTest.CoreIntegration
         [Fact]
         public void CanBuildAndQuery()
         {
-            var folder = TestData.Get(@"TestData\ExampleExtension");
+            var folder = TestData.Get("TestData", "ExampleExtension");
             var build = new Builder(folder, typeof(ExampleExtensionFactory), new[] { Path.Combine(folder, "data") });
 
             var results = build.BuildAndQuery(Build, "Wix4Example");
@@ -58,7 +58,7 @@ namespace WixToolsetTest.CoreIntegration
         [Fact]
         public void CanBuildWithExampleExtension()
         {
-            var folder = TestData.Get(@"TestData\ExampleExtension");
+            var folder = TestData.Get("TestData", "ExampleExtension");
 
             using (var fs = new DisposableFileSystem())
             {
@@ -157,6 +157,65 @@ namespace WixToolsetTest.CoreIntegration
         }
 
         [Fact]
+        public void CanBuildWithExampleExtensionLocalizedDefault()
+        {
+            var folder = TestData.Get("TestData", "ExampleExtensionLocalized");
+
+            using (var fs = new DisposableFileSystem())
+            {
+                var intermediateFolder = fs.GetFolder();
+
+                var result = WixRunner.Execute(new[]
+                {
+                    "build",
+                    Path.Combine(folder, "LocalizedPackage.wxs"),
+                    "-ext", ExtensionPaths.ExampleExtensionPath,
+                    "-bindpath", Path.Combine(folder, "data"),
+                    "-intermediateFolder", intermediateFolder,
+                    "-o", Path.Combine(intermediateFolder, "bin", "extest.msi")
+                });
+
+                result.AssertSuccess();
+
+                var pdb = Intermediate.Load(Path.Combine(intermediateFolder, "bin", "extest.wixpdb"));
+                var section = pdb.Sections.Single();
+
+                var property = section.Symbols.OfType<PropertySymbol>().Single(t => t.Id.Id == "LocalizedProperty");
+                WixAssert.StringEqual("en-us", property.Value);
+            }
+        }
+
+        [Fact]
+        public void CanBuildWithExampleExtensionLocalizedNonDefault()
+        {
+            var folder = TestData.Get("TestData", "ExampleExtensionLocalized");
+
+            using (var fs = new DisposableFileSystem())
+            {
+                var intermediateFolder = fs.GetFolder();
+
+                var result = WixRunner.Execute(new[]
+                {
+                    "build",
+                    Path.Combine(folder, "LocalizedPackage.wxs"),
+                    "-culture", "ja-jp",
+                    "-ext", ExtensionPaths.ExampleExtensionPath,
+                    "-bindpath", Path.Combine(folder, "data"),
+                    "-intermediateFolder", intermediateFolder,
+                    "-o", Path.Combine(intermediateFolder, "bin", "extest.msi")
+                });
+
+                result.AssertSuccess();
+
+                var pdb = Intermediate.Load(Path.Combine(intermediateFolder, "bin", "extest.wixpdb"));
+                var section = pdb.Sections.Single();
+
+                var property = section.Symbols.OfType<PropertySymbol>().Single(t => t.Id.Id == "LocalizedProperty");
+                WixAssert.StringEqual("ja-jp", property.Value);
+            }
+        }
+
+        [Fact]
         public void CanParseCommandLineWithExtension()
         {
             var folder = TestData.Get(@"TestData\ExampleExtension");
@@ -192,7 +251,7 @@ namespace WixToolsetTest.CoreIntegration
         [Fact]
         public void CannotBuildWithMissingExtension()
         {
-            var folder = TestData.Get(@"TestData\ExampleExtension");
+            var folder = TestData.Get("TestData", "ExampleExtension");
 
             using (var fs = new DisposableFileSystem())
             {
@@ -213,7 +272,7 @@ namespace WixToolsetTest.CoreIntegration
         [Fact]
         public void CannotBuildWithMissingVersionedExtension()
         {
-            var folder = TestData.Get(@"TestData\ExampleExtension");
+            var folder = TestData.Get("TestData", "ExampleExtension");
 
             using (var fs = new DisposableFileSystem())
             {
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/LocalizedPackage.wxs b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/LocalizedPackage.wxs
new file mode 100644
index 00000000..646c0875
--- /dev/null
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/LocalizedPackage.wxs
@@ -0,0 +1,18 @@
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
+  <Package Name="LocalizedPackage" Version="1.0.0.0" Manufacturer="Example Corporation" UpgradeCode="047730a5-30fe-4a62-a520-da9381b8226a" Compressed="no" Scope="perMachine">
+
+    <PropertyRef Id="LocalizedProperty" />
+
+    <Feature Id="ProductFeature">
+        <Component Directory="INSTALLFOLDER">
+            <File Source="example.txt" />
+        </Component>
+    </Feature>
+  </Package>
+
+  <Fragment>
+    <StandardDirectory Id="ProgramFilesFolder">
+      <Directory Id="INSTALLFOLDER" Name="MsiPackage" />
+    </StandardDirectory>
+  </Fragment>
+</Wix>
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/PackageComponents.wxs b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/PackageComponents.wxs
new file mode 100644
index 00000000..7f17b538
--- /dev/null
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/PackageComponents.wxs
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs"
+     xmlns:ex="http://www.example.com/scheams/v1/wxs">
+  <Fragment>
+    <ComponentGroup Id="ProductComponents" Directory="INSTALLFOLDER">
+       <Component>
+         <File Source="example.txt" />
+         <ex:Example Id="Foo" Value="Bar" />
+       </Component>
+    </ComponentGroup>
+  </Fragment>
+</Wix>
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/data/example.txt b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/data/example.txt
new file mode 100644
index 00000000..1b4ffe8a
--- /dev/null
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/ExampleExtensionLocalized/data/example.txt
@@ -0,0 +1 @@
+This is example.txt.
\ No newline at end of file