Ensures all back office controllers have the thread culture set, we do this with a new application model targeting all back office controllers.

src/Umbraco.Web.Common/ApplicationModels/BackOfficeApplicationModelProvider.cs

@@ -0,0 +1,58 @@
+using Microsoft.AspNetCore.Mvc.ApplicationModels;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using System.Collections.Generic;
+using System.Linq;
+using Umbraco.Web.Common.Attributes;
+
+namespace Umbraco.Web.Common.ApplicationModels
+{
+    /// <summary>
+    /// An application model provider for all Umbraco Back Office controllers
+    /// </summary>
+    public class BackOfficeApplicationModelProvider : IApplicationModelProvider
+    {
+        public BackOfficeApplicationModelProvider(IModelMetadataProvider modelMetadataProvider)
+        {
+            ActionModelConventions = new List<IActionModelConvention>()
+            {         
+                new BackOfficeIdentityCultureConvention()
+            };
+        }
+
+        /// <summary>
+        /// Will execute after <see cref="DefaultApplicationModelProvider"/>
+        /// </summary>
+        public int Order => 0;
+
+        public List<IActionModelConvention> ActionModelConventions { get; }
+
+        public void OnProvidersExecuted(ApplicationModelProviderContext context)
+        {
+        }
+
+        public void OnProvidersExecuting(ApplicationModelProviderContext context)
+        {
+            foreach (var controller in context.Result.Controllers)
+            {
+                if (!IsBackOfficeController(controller))
+                    continue;
+
+                foreach (var action in controller.Actions)
+                {
+                    foreach (var convention in ActionModelConventions)
+                    {
+                        convention.Apply(action);
+                    }
+                }
+
+            }
+        }
+
+        private bool IsBackOfficeController(ControllerModel controller)
+        {
+            var pluginControllerAttribute = controller.Attributes.OfType<PluginControllerAttribute>().FirstOrDefault();
+            return pluginControllerAttribute != null
+                && pluginControllerAttribute.AreaName == Core.Constants.Web.Mvc.BackOfficeArea;
+        }
+    }
+}

src/Umbraco.Web.Common/ApplicationModels/BackOfficeIdentityCultureConvention.cs

@@ -0,0 +1,13 @@
+using Microsoft.AspNetCore.Mvc.ApplicationModels;
+using Umbraco.Web.Common.Filters;
+
+namespace Umbraco.Web.Common.ApplicationModels
+{
+    public class BackOfficeIdentityCultureConvention : IActionModelConvention
+    {
+        public void Apply(ActionModel action)
+        {
+            action.Filters.Add(new BackOfficeCultureFilter());
+        }
+    }
+}

src/Umbraco.Web.Common/ApplicationModels/UmbracoApiBehaviorApplicationModelProvider.cs

@@ -10,7 +10,7 @@ namespace Umbraco.Web.Common.ApplicationModels
 {
 
     /// <summary>
-    /// A custom application model provider for Umbraco controllers
+    /// An application model provider for Umbraco API controllers to behave like WebApi controllers
     /// </summary>
     /// <remarks>
     /// <para>

src/Umbraco.Web.Common/Extensions/UmbracoCoreServiceCollectionExtensions.cs

@@ -156,24 +156,7 @@ namespace Umbraco.Extensions
                 out factory);
 
             return services;
-        }
-
-        public static IServiceCollection AddUmbracoRequestLocalization(this IServiceCollection services)
-        {
-            services.Configure<RequestLocalizationOptions>(options =>
-            {
-                var supportedCultures = CultureInfo
-                    .GetCultures(CultureTypes.AllCultures & ~ CultureTypes.NeutralCultures)
-                    .Where(cul => !String.IsNullOrEmpty(cul.Name))
-                    .ToArray();
-
-                options.SupportedCultures = supportedCultures;
-                options.SupportedUICultures = supportedCultures;
-                options.AddInitialRequestCultureProvider(new UmbracoBackOfficeIdentityCultureProvider());
-            });
-
-            return services;
-        }
+        }   
 
         /// <summary>
         /// Adds the Umbraco Back Core requirements
@@ -201,9 +184,6 @@ namespace Umbraco.Extensions
             if (container is null) throw new ArgumentNullException(nameof(container));
             if (entryAssembly is null) throw new ArgumentNullException(nameof(entryAssembly));
 
-            // Set culture options
-            services.AddUmbracoRequestLocalization();
-
             // Add supported databases
             services.AddUmbracoSqlCeSupport();
             services.AddUmbracoSqlServerSupport();

src/Umbraco.Web.Common/Extensions/UmbracoWebServiceCollectionExtensions.cs

@@ -36,6 +36,7 @@ namespace Umbraco.Extensions
         {
             services.ConfigureOptions<UmbracoMvcConfigureOptions>();
             services.TryAddEnumerable(ServiceDescriptor.Transient<IApplicationModelProvider, UmbracoApiBehaviorApplicationModelProvider>());
+            services.TryAddEnumerable(ServiceDescriptor.Transient<IApplicationModelProvider, BackOfficeApplicationModelProvider>());
 
             // TODO: We need to avoid this, surely there's a way? See ContainerTests.BuildServiceProvider_Before_Host_Is_Configured
             var serviceProvider = services.BuildServiceProvider();

src/Umbraco.Web.Common/Filters/BackOfficeCultureFilter.cs

@@ -0,0 +1,34 @@
+using System;
+using Microsoft.AspNetCore.Mvc.Filters;
+using Umbraco.Core.Security;
+using System.Globalization;
+
+namespace Umbraco.Web.Common.Filters
+{
+    /// <summary>
+    /// Applied to all Umbraco controllers to ensure the thread culture is set to the culture assigned to the back office identity
+    /// </summary>
+    public class BackOfficeCultureFilter : IActionFilter
+    {
+        public void OnActionExecuted(ActionExecutedContext context)
+        {
+        }
+
+        public void OnActionExecuting(ActionExecutingContext context)
+        {
+            var culture = context.HttpContext.User.Identity.GetCulture();
+            if (culture != null)
+            {
+                SetCurrentThreadCulture(culture);
+            }
+        }
+
+        private static void SetCurrentThreadCulture(CultureInfo culture)
+        {
+            CultureInfo.CurrentCulture = culture;
+            CultureInfo.CurrentUICulture = culture;
+        }
+    }
+
+
+}