Fixes #3042

Code comes from v8, commit: 9bfe9e6bbf

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;
@@ -458,5 +460,23 @@ 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

@@ -102,7 +102,8 @@ 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);
 
@@ -120,21 +121,5 @@ namespace Umbraco.Core.Security
             if (_appCtx.IsConfigured && _appCtx.IsUpgrading == false)
                 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

@@ -509,6 +509,13 @@ 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;