Ensures that the thread culture is set both in owin and in aspnet

src/Umbraco.Core/Security/AuthenticationExtensions.cs

@@ -1,8 +1,10 @@
 using System;
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
+using System.Globalization;
 using System.Linq;
 using System.Net.Http;
 using System.Net.Http.Headers;
@@ -441,5 +443,24 @@ namespace Umbraco.Core.Security
 
             return ticket;
         }
+
+        /// <summary>
+        /// Ensures that the thread culture is set based on the back office user's culture
+        /// </summary>
+        /// <param name="identity"></param>
+        internal static void EnsureCulture(this IIdentity identity)
+        {
+            if (identity is UmbracoBackOfficeIdentity umbIdentity && umbIdentity.IsAuthenticated)
+            {
+                Thread.CurrentThread.CurrentUICulture =
+                    Thread.CurrentThread.CurrentCulture =
+                        UserCultures.GetOrAdd(umbIdentity.Culture, s => new CultureInfo(s));
+            }
+        }
+
+        /// <summary>
+        /// Used so that we aren't creating a new CultureInfo object for every single request
+        /// </summary>
+        private static readonly ConcurrentDictionary<string, CultureInfo> UserCultures = new ConcurrentDictionary<string, CultureInfo>();
     }
 }

src/Umbraco.Core/Security/BackOfficeCookieAuthenticationProvider.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Globalization;
+using System.Runtime.CompilerServices;
 using System.Security.Claims;
 using System.Threading;
 using System.Threading.Tasks;
@@ -81,7 +82,8 @@ namespace Umbraco.Core.Security
         /// <returns/>
         public override async Task ValidateIdentity(CookieValidateIdentityContext context)
         {
-            EnsureCulture(context);
+            //ensure the thread culture is set
+            context?.Identity?.EnsureCulture();
 
             await EnsureValidSessionId(context);
 
@@ -100,20 +102,8 @@ namespace Umbraco.Core.Security
                 await SessionIdValidator.ValidateSessionAsync(TimeSpan.FromMinutes(1), context);
         }
 
-        private void EnsureCulture(CookieValidateIdentityContext context)
-        {
-            var umbIdentity = context.Identity as UmbracoBackOfficeIdentity;
-            if (umbIdentity != null && umbIdentity.IsAuthenticated)
-            {
-                Thread.CurrentThread.CurrentCulture =
-                    Thread.CurrentThread.CurrentUICulture =
-                        UserCultures.GetOrAdd(umbIdentity.Culture, s => new CultureInfo(s));
-            }
-        }
         
-        /// <summary>
-        /// Used so that we aren't creating a new CultureInfo object for every single request
-        /// </summary>
-        private static readonly ConcurrentDictionary<string, CultureInfo> UserCultures = new ConcurrentDictionary<string, CultureInfo>();
+
+        
     }
 }

src/Umbraco.Web/UmbracoModule.cs

@@ -1,8 +1,10 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
 using System.Text;
+using System.Threading;
 using System.Web;
 using System.Web.Routing;
 using LightInject;
@@ -15,6 +17,7 @@ using Umbraco.Web.Security;
 using Umbraco.Core.Collections;
 using Umbraco.Core.Exceptions;
 using Umbraco.Core.Persistence;
+using Umbraco.Core.Security;
 using Umbraco.Core.Services;
 using Umbraco.Web.Composing;
 using Umbraco.Web.PublishedCache;
@@ -562,6 +565,13 @@ namespace Umbraco.Web
                 }
             };
 
+            app.PostAuthenticateRequest += (sender, e) =>
+            {
+                var httpContext = ((HttpApplication) sender).Context;
+                //ensure the thread culture is set
+                httpContext.User?.Identity?.EnsureCulture();
+            };
+
             app.PostResolveRequestCache += (sender, e) =>
             {
                 var httpContext = ((HttpApplication) sender).Context;