Ensures the culture is set for requests for both front-end and back office

src/Umbraco.Web.Common/Localization/UmbracoRequestLocalizationOptions.cs

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+using System.Collections.Generic;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Localization;
+using Microsoft.Extensions.Options;
+using Umbraco.Core.Configuration.Models;
+
+namespace Umbraco.Web.Common.Localization
+{
+    /// <summary>
+    /// Custom Umbraco options configuration for <see cref="RequestLocalizationOptions"/>
+    /// </summary>
+    public class UmbracoRequestLocalizationOptions : IConfigureOptions<RequestLocalizationOptions>
+    {
+        private readonly IOptions<GlobalSettings> _globalSettings;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UmbracoRequestLocalizationOptions"/> class.
+        /// </summary>
+        public UmbracoRequestLocalizationOptions(IOptions<GlobalSettings> globalSettings) => _globalSettings = globalSettings;
+
+        /// <inheritdoc/>
+        public void Configure(RequestLocalizationOptions options)
+        {
+            // set the default culture to what is in config
+            options.DefaultRequestCulture = new RequestCulture(_globalSettings.Value.DefaultUILanguage);
+
+            // add a custom provider
+            if (options.RequestCultureProviders == null)
+            {
+                options.RequestCultureProviders = new List<IRequestCultureProvider>();
+            }
+
+            options.RequestCultureProviders.Insert(0, new UmbracoBackOfficeIdentityCultureProvider());
+            options.RequestCultureProviders.Insert(1, new UmbracoPublishedContentCultureProvider());
+        }
+    }
+}