Use CurrentCulture as the page language.

framework/src/Volo.Abp.AspNetCore.Mvc.UI.Theme.Basic/Themes/Basic/Layouts/Application.cshtml

@@ -1,4 +1,4 @@
-@using Volo.Abp.AspNetCore.Mvc.AntiForgery
+@using Volo.Abp.AspNetCore.Mvc.AntiForgery
 @using Volo.Abp.AspNetCore.Mvc.UI.Components.LayoutHook
 @using Volo.Abp.AspNetCore.Mvc.UI.Layout
 @using Volo.Abp.AspNetCore.Mvc.UI.Theme.Basic.Bundling
@@ -25,17 +25,11 @@
 
         pageTitle = PageLayout.Content.Title + pageTitle;
     }
-
-    var pageLang = ViewBag.Lang == null ? "" : $" lang={ViewBag.Lang}";
-    if (PageLayout.Content.Lang != null)
-    {
-        pageLang = $" lang={PageLayout.Content.Lang}";
-    }
 }
 
 <!DOCTYPE html>
 
-<@($"html{pageLang}")>
+<html lang="@CultureInfo.CurrentCulture.Name">
 <head>
     @await Component.InvokeLayoutHookAsync(LayoutHooks.Head.First, StandardLayouts.Application)
 

framework/src/Volo.Abp.AspNetCore.Mvc.UI.Theme.Basic/Themes/Basic/Layouts/Empty.cshtml

@@ -1,4 +1,4 @@
-@using Volo.Abp.AspNetCore.Mvc.AntiForgery
+@using Volo.Abp.AspNetCore.Mvc.AntiForgery
 @using Volo.Abp.AspNetCore.Mvc.UI.Components.LayoutHook
 @using Volo.Abp.AspNetCore.Mvc.UI.Layout
 @using Volo.Abp.AspNetCore.Mvc.UI.Theme.Basic.Bundling
@@ -24,17 +24,11 @@
 
         pageTitle = PageLayout.Content.Title + pageTitle;
     }
-
-    var pageLang = ViewBag.Lang == null ? "" : $" lang={ViewBag.Lang}";
-    if (PageLayout.Content.Lang != null)
-    {
-        pageLang = $" lang={PageLayout.Content.Lang}";
-    }
 }
 
 <!DOCTYPE html>
 
-<@($"html{pageLang}")>
+<html lang="@CultureInfo.CurrentCulture.Name">
 <head>
     @await Component.InvokeLayoutHookAsync(LayoutHooks.Head.First, StandardLayouts.Empty)
 

framework/src/Volo.Abp.AspNetCore.Mvc.UI.Theme.Basic/Themes/Basic/_ViewImports.cshtml

@@ -1,3 +1,4 @@
-@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+@using System.Globalization
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
 @addTagHelper *, Volo.Abp.AspNetCore.Mvc.UI.Bootstrap
 @addTagHelper *, Volo.Abp.AspNetCore.Mvc.UI.Bundling

framework/src/Volo.Abp.AspNetCore.Mvc.UI/Volo/Abp/AspNetCore/Mvc/UI/Layout/ContentLayout.cs

@@ -5,8 +5,6 @@ namespace Volo.Abp.AspNetCore.Mvc.UI.Layout
 {
     public class ContentLayout
     {
-        public string Lang { get; set; }
-
         public string Title { get; set; }
 
         public BreadCrumb BreadCrumb { get; }

modules/docs/src/Volo.Docs.Web/Pages/Documents/Project/Index.cshtml

@@ -17,7 +17,6 @@
 @{
     ViewBag.FluidLayout = true;
     Layout = ThemeManager.CurrentTheme.GetEmptyLayout();
-    PageLayout.Content.Lang = Model.LanguageCode;
     PageLayout.Content.Title = Model.DocumentName?.Replace("-", " ");
 }
 @section styles {