diff --git a/src/Microsoft.AspNetCore.Localization/ProviderCultureResult.cs b/src/Microsoft.AspNetCore.Localization/ProviderCultureResult.cs
index a8fa269ff3..2cb8b50e71 100644
--- a/src/Microsoft.AspNetCore.Localization/ProviderCultureResult.cs
+++ b/src/Microsoft.AspNetCore.Localization/ProviderCultureResult.cs
@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information. 
 
 using System.Collections.Generic;
+using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNetCore.Localization
 {
@@ -15,8 +16,8 @@ namespace Microsoft.AspNetCore.Localization
         /// <see cref="UICultures"/> properties set to the same culture value.
         /// </summary>
         /// <param name="culture">The name of the culture to be used for formatting, text, i.e. language.</param>
-        public ProviderCultureResult(string culture)
-            : this(new List<string> { culture }, new List<string> { culture })
+        public ProviderCultureResult(StringSegment culture)
+            : this(new List<StringSegment> { culture }, new List<StringSegment> { culture })
         {
         }
 
@@ -26,8 +27,8 @@ namespace Microsoft.AspNetCore.Localization
         /// </summary>
         /// <param name="culture">The name of the culture to be used for formatting.</param>
         /// <param name="uiCulture"> The name of the ui culture to be used for text, i.e. language.</param>
-        public ProviderCultureResult(string culture, string uiCulture)
-            : this(new List<string> { culture }, new List<string> { uiCulture })
+        public ProviderCultureResult(StringSegment culture, StringSegment uiCulture)
+            : this(new List<StringSegment> { culture }, new List<StringSegment> { uiCulture })
         {
         }
 
@@ -36,7 +37,7 @@ namespace Microsoft.AspNetCore.Localization
         /// <see cref="UICultures"/> properties set to the same culture value.
         /// </summary>
         /// <param name="cultures">The list of cultures to be used for formatting, text, i.e. language.</param>
-        public ProviderCultureResult(IList<string> cultures)
+        public ProviderCultureResult(IList<StringSegment> cultures)
             : this(cultures, cultures)
         {
         }
@@ -47,7 +48,7 @@ namespace Microsoft.AspNetCore.Localization
         /// </summary>
         /// <param name="cultures">The list of cultures to be used for formatting.</param>
         /// <param name="uiCultures">The list of ui cultures to be used for text, i.e. language.</param>
-        public ProviderCultureResult(IList<string> cultures, IList<string> uiCultures)
+        public ProviderCultureResult(IList<StringSegment> cultures, IList<StringSegment> uiCultures)
         {
             Cultures = cultures;
             UICultures = uiCultures;
@@ -56,11 +57,11 @@ namespace Microsoft.AspNetCore.Localization
         /// <summary>
         /// Gets the list of cultures to be used for formatting.
         /// </summary>
-        public IList<string> Cultures { get; }
+        public IList<StringSegment> Cultures { get; }
 
         /// <summary>
         /// Gets the list of ui cultures to be used for text, i.e. language;
         /// </summary>
-        public IList<string> UICultures { get; }
+        public IList<StringSegment> UICultures { get; }
     }
 }
diff --git a/src/Microsoft.AspNetCore.Localization/RequestLocalizationMiddleware.cs b/src/Microsoft.AspNetCore.Localization/RequestLocalizationMiddleware.cs
index 0e34da2231..01cef8c4e3 100644
--- a/src/Microsoft.AspNetCore.Localization/RequestLocalizationMiddleware.cs
+++ b/src/Microsoft.AspNetCore.Localization/RequestLocalizationMiddleware.cs
@@ -10,6 +10,7 @@ using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Options;
+using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNetCore.Localization
 {
@@ -131,7 +132,7 @@ namespace Microsoft.AspNetCore.Localization
         }
 
         private static CultureInfo GetCultureInfo(
-            IList<string> cultureNames,
+            IList<StringSegment> cultureNames,
             IList<CultureInfo> supportedCultures,
             bool fallbackToParentCultures)
         {
@@ -152,7 +153,7 @@ namespace Microsoft.AspNetCore.Localization
             return null;
         }
 
-        private static CultureInfo GetCultureInfo(string name, IList<CultureInfo> supportedCultures)
+        private static CultureInfo GetCultureInfo(StringSegment name, IList<CultureInfo> supportedCultures)
         {
             // Allow only known culture names as this API is called with input from users (HTTP requests) and
             // creating CultureInfo objects is expensive and we don't want it to throw either.
@@ -161,7 +162,7 @@ namespace Microsoft.AspNetCore.Localization
                 return null;
             }
             var culture = supportedCultures.FirstOrDefault(
-                supportedCulture => string.Equals(supportedCulture.Name, name, StringComparison.OrdinalIgnoreCase));
+                supportedCulture => StringSegment.Equals(supportedCulture.Name, name, StringComparison.OrdinalIgnoreCase));
 
             if (culture == null)
             {
@@ -172,7 +173,7 @@ namespace Microsoft.AspNetCore.Localization
         }
 
         private static CultureInfo GetCultureInfo(
-            string cultureName,
+            StringSegment cultureName,
             IList<CultureInfo> supportedCultures,
             bool fallbackToParentCultures,
             int currentDepth)
@@ -186,7 +187,7 @@ namespace Microsoft.AspNetCore.Localization
                 if (lastIndexOfHyphen > 0)
                 {
                     // Trim the trailing section from the culture name, e.g. "fr-FR" becomes "fr"
-                    var parentCultureName = cultureName.Substring(0, lastIndexOfHyphen);
+                    var parentCultureName = cultureName.Subsegment(0, lastIndexOfHyphen);
 
                     culture = GetCultureInfo(parentCultureName, supportedCultures, fallbackToParentCultures, currentDepth + 1);
                 }
diff --git a/src/Microsoft.AspNetCore.Localization/breakingchanges.netcore.json b/src/Microsoft.AspNetCore.Localization/breakingchanges.netcore.json
new file mode 100644
index 0000000000..5568b582c6
--- /dev/null
+++ b/src/Microsoft.AspNetCore.Localization/breakingchanges.netcore.json
@@ -0,0 +1,32 @@
+[
+      {
+        "TypeId": "public class Microsoft.AspNetCore.Localization.ProviderCultureResult",
+        "MemberId": "public .ctor(System.Collections.Generic.IList<System.String> cultures)",
+        "Kind": "Removal"
+      },
+      {
+        "TypeId": "public class Microsoft.AspNetCore.Localization.ProviderCultureResult",
+        "MemberId": "public .ctor(System.Collections.Generic.IList<System.String> cultures, System.Collections.Generic.IList<System.String> uiCultures)",
+        "Kind": "Removal"
+      },
+      {
+        "TypeId": "public class Microsoft.AspNetCore.Localization.ProviderCultureResult",
+        "MemberId": "public .ctor(System.String culture)",
+        "Kind": "Removal"
+      },
+      {
+        "TypeId": "public class Microsoft.AspNetCore.Localization.ProviderCultureResult",
+        "MemberId": "public .ctor(System.String culture, System.String uiCulture)",
+        "Kind": "Removal"
+      },
+      {
+        "TypeId": "public class Microsoft.AspNetCore.Localization.ProviderCultureResult",
+        "MemberId": "public System.Collections.Generic.IList<System.String> get_Cultures()",
+        "Kind": "Removal"
+      },
+      {
+        "TypeId": "public class Microsoft.AspNetCore.Localization.ProviderCultureResult",
+        "MemberId": "public System.Collections.Generic.IList<System.String> get_UICultures()",
+        "Kind": "Removal"
+      }
+]
\ No newline at end of file