Add language selection and system display language support

Introduces a LanguageHelper utility to manage application language settings, preferring system display language over regional settings. Adds a language selection UI to Settings, persists user choice, and applies language overrides at startup and view creation. Updates LocalizationService to use display language when no override is set.

src/CalcViewModel/Common/LocalizationService.cpp

@@ -74,8 +74,27 @@ void LocalizationService::OverrideWithLanguage(_In_ const wchar_t* const languag
 /// <param name="overridedLanguage">RFC-5646 identifier of the language to use, if null, will use the current language of the system</param>
 LocalizationService::LocalizationService(_In_ const wchar_t * const overridedLanguage)
 {
+    using namespace Windows::System::UserProfile;
+
     m_isLanguageOverrided = overridedLanguage != nullptr;
-    m_language = m_isLanguageOverrided ? ref new Platform::String(overridedLanguage) : ApplicationLanguages::Languages->GetAt(0);
+    if (m_isLanguageOverrided)
+    {
+        m_language = ref new Platform::String(overridedLanguage);
+    }
+    else
+    {
+        // Prefer the system Display Language over Regional Settings
+        auto displayLanguages = GlobalizationPreferences::Languages;
+        if (displayLanguages != nullptr && displayLanguages->Size > 0)
+        {
+            m_language = ref new Platform::String(displayLanguages->GetAt(0)->Data());
+        }
+        else
+        {
+            // Fallback to the default application language list
+            m_language = ApplicationLanguages::Languages->GetAt(0);
+        }
+    }
     m_flowDirection = ResourceContext::GetForViewIndependentUse()->QualifierValues->Lookup(L"LayoutDirection")
         != L"LTR" ? FlowDirection::RightToLeft : FlowDirection::LeftToRight;
     wstring localeName = wstring(m_language->Data());

src/Calculator/App.xaml.cs

@@ -35,6 +35,20 @@ namespace CalculatorApp
         /// </summary>
         public App()
         {
+            // Ensure we choose the UI language based on System Display Language
+            // before any resources are loaded or singletons are created.
+            // If user selected a forced language, apply it; otherwise use Display Language
+            var selected = LanguageHelper.SelectedLanguage;
+            if (!string.IsNullOrEmpty(selected) && !string.Equals(selected, "system", StringComparison.OrdinalIgnoreCase))
+            {
+                LanguageHelper.ApplyUserLanguageSelection(selected);
+            }
+            else
+            {
+                LanguageHelper.InitializeDisplayLanguage();
+            }
+            LanguageHelper.LogLanguageDebugInfo(); // Debug
+
             InitializeComponent();
             NarratorNotifier.RegisterDependencyProperties();
 
@@ -149,6 +163,18 @@ namespace CalculatorApp
             // Place the frame in the current Window
             Window.Current.Content = rootFrame;
             ThemeHelper.InitializeAppTheme();
+
+            // Initialize Calculator to use System Display Language instead of Regional Settings
+            // Re-apply language at view creation based on persisted selection
+            var selected = LanguageHelper.SelectedLanguage;
+            var effective = selected == "system" ? LanguageHelper.GetSystemDisplayLanguage() : selected;
+            if (!string.IsNullOrEmpty(effective))
+            {
+                LanguageHelper.ApplyViewResourceLanguages(effective);
+                LanguageHelper.ApplyGlobalResourceLanguages(effective);
+            }
+            LanguageHelper.LogLanguageDebugInfo(); // Debug
+
             Window.Current.Activate();
         }
 

src/Calculator/Utils/LanguageHelper.cs

@@ -0,0 +1,256 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using Windows.Storage;
+
+namespace CalculatorApp.Utils
+{
+    /// <summary>
+    /// Helper class for managing application language settings
+    /// </summary>
+    public static class LanguageHelper
+    {
+        private const string SelectedLanguageKey = "SelectedLanguage"; // "system" or BCP-47 tag
+
+        /// <summary>
+        /// Initialize the application to use the System Display Language instead of Regional Settings
+        /// </summary>
+        public static void InitializeDisplayLanguage()
+        {
+            try
+            {
+                var displayLanguage = GetSystemDisplayLanguage();
+                if (!string.IsNullOrEmpty(displayLanguage))
+                {
+                    Debug.WriteLine($"Setting Calculator to use Display Language: {displayLanguage}");
+                    SetApplicationLanguage(displayLanguage);
+                }
+                else
+                {
+                    Debug.WriteLine("Could not detect system display language, using default behavior");
+                }
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error initializing display language: {ex.Message}");
+                // Don't throw - let the app continue with default behavior
+            }
+        }
+
+        /// <summary>
+        /// Persisted selected language code. "system" means follow system (Display Language)
+        /// </summary>
+        public static string SelectedLanguage
+        {
+            get => ApplicationData.Current.LocalSettings.Values[SelectedLanguageKey]?.ToString() ?? "system";
+            set => ApplicationData.Current.LocalSettings.Values[SelectedLanguageKey] = value;
+        }
+
+        /// <summary>
+        /// Return available UI languages for selection (System Default + supported app languages)
+        /// </summary>
+        public static List<LanguageInfo> GetAvailableLanguages()
+        {
+            var list = new List<LanguageInfo>();
+            list.Add(new LanguageInfo { Code = "system", NativeName = "System Default" });
+
+            foreach (var code in GetSupportedLanguageCodes())
+            {
+                try
+                {
+                    var lang = new Windows.Globalization.Language(code);
+                    list.Add(new LanguageInfo { Code = code, NativeName = lang.NativeName });
+                }
+                catch
+                {
+                    list.Add(new LanguageInfo { Code = code, NativeName = code });
+                }
+            }
+
+            return list;
+        }
+
+        private static IEnumerable<string> GetSupportedLanguageCodes()
+        {
+            // Full list based on src/Calculator/Resources/* folders
+            return new[]
+            {
+                "af-ZA","am-ET","ar-SA","az-Latn-AZ","bg-BG","ca-ES","cs-CZ","da-DK","de-DE","el-GR",
+                "en-GB","en-US","es-ES","es-MX","et-EE","eu-ES","fa-IR","fi-FI","fil-PH","fr-CA","fr-FR",
+                "gl-ES","he-IL","hi-IN","hr-HR","hu-HU","id-ID","is-IS","it-IT","ja-JP","kk-KZ","km-KH",
+                "kn-IN","ko-KR","lo-LA","lt-LT","lv-LV","mk-MK","ml-IN","ms-MY","nb-NO","nl-NL","pl-PL",
+                "pt-BR","pt-PT","ro-RO","ru-RU","sk-SK","sl-SI","sq-AL","sr-Latn-RS","sv-SE","ta-IN",
+                "te-IN","th-TH","tr-TR","uk-UA","vi-VN","zh-CN","zh-TW"
+            };
+        }
+
+        /// <summary>
+        /// Apply a user-selected language code (or "system") immediately and persist preference
+        /// </summary>
+        public static void ApplyUserLanguageSelection(string code)
+        {
+            SelectedLanguage = code;
+
+            string languageToUse = code == "system" ? GetSystemDisplayLanguage() : code;
+            if (string.IsNullOrEmpty(languageToUse))
+            {
+                Debug.WriteLine("No language resolved; skipping override");
+                return;
+            }
+
+            SetApplicationLanguage(languageToUse);
+            ApplyViewResourceLanguages(languageToUse);
+            ApplyGlobalResourceLanguages(languageToUse);
+        }
+
+        /// <summary>
+        /// Gets the system's Display Language (not Regional Settings)
+        /// </summary>
+        /// <returns>The primary display language code (e.g., "en-US", "ja-JP")</returns>
+        public static string GetSystemDisplayLanguage()
+        {
+            try
+            {
+                // Primary method: Get the user's preferred UI languages from GlobalizationPreferences
+                // This returns the Display Language, not Regional Settings
+                var userLanguages = Windows.System.UserProfile.GlobalizationPreferences.Languages;
+                if (userLanguages.Count > 0)
+                {
+                    var primaryLanguage = userLanguages[0];
+                    Debug.WriteLine($"Display Language from GlobalizationPreferences: {primaryLanguage}");
+                    return primaryLanguage;
+                }
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error getting GlobalizationPreferences: {ex.Message}");
+                
+                // Fallback method: Try ResourceContext
+                try
+                {
+                    var resourceContext = Windows.ApplicationModel.Resources.Core.ResourceContext.GetForCurrentView();
+                    if (resourceContext.Languages.Count > 0)
+                    {
+                        var primaryLanguage = resourceContext.Languages[0];
+                        Debug.WriteLine($"Display Language from ResourceContext: {primaryLanguage}");
+                        return primaryLanguage;
+                    }
+                }
+                catch (Exception ex2)
+                {
+                    Debug.WriteLine($"Error getting ResourceContext languages: {ex2.Message}");
+                }
+            }
+            
+            return null;
+        }
+
+        /// <summary>
+        /// Sets the application language using ApplicationLanguages.PrimaryLanguageOverride
+        /// </summary>
+        /// <param name="languageCode">Language code (e.g., "en-US", "ja-JP")</param>
+        public static void SetApplicationLanguage(string languageCode)
+        {
+            try
+            {
+                // Set the primary language override for this application
+                // This overrides the default language resolution and forces the app to use the specified language
+                Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride = languageCode;
+                Debug.WriteLine($"ApplicationLanguages.PrimaryLanguageOverride set to: {languageCode}");
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error setting application language to {languageCode}: {ex.Message}");
+                throw;
+            }
+        }
+
+        /// <summary>
+        /// Apply the specified language to ResourceContext used outside of any view
+        /// </summary>
+        public static void ApplyGlobalResourceLanguages(string languageCode)
+        {
+            try
+            {
+                // Set process-wide qualifier for language
+                Windows.ApplicationModel.Resources.Core.ResourceContext.SetGlobalQualifierValue("Language", languageCode);
+                Debug.WriteLine($"Global qualifier 'Language' set to: {languageCode}");
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error applying global resource language: {ex.Message}");
+            }
+        }
+
+        /// <summary>
+        /// Apply the specified language to ResourceContext for the current view
+        /// </summary>
+        public static void ApplyViewResourceLanguages(string languageCode)
+        {
+            try
+            {
+                var ctx = Windows.ApplicationModel.Resources.Core.ResourceContext.GetForCurrentView();
+                // Set qualifier value for this view
+                ctx.QualifierValues["Language"] = languageCode;
+                Debug.WriteLine($"View qualifier 'Language' set to: {languageCode}");
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error applying view resource language: {ex.Message}");
+            }
+        }
+
+        /// <summary>
+        /// Clears the application language override (for debugging/testing)
+        /// </summary>
+        public static void ClearApplicationLanguageOverride()
+        {
+            try
+            {
+                Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride = "";
+                Debug.WriteLine("ApplicationLanguages.PrimaryLanguageOverride cleared");
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error clearing application language override: {ex.Message}");
+            }
+        }
+
+        /// <summary>
+        /// Gets debug information about current language settings
+        /// </summary>
+        public static void LogLanguageDebugInfo()
+        {
+            try
+            {
+                var currentOverride = Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride;
+                var systemLanguages = Windows.System.UserProfile.GlobalizationPreferences.Languages;
+                var appLanguages = Windows.Globalization.ApplicationLanguages.Languages;
+
+                Debug.WriteLine("=== Language Debug Info ===");
+                Debug.WriteLine($"PrimaryLanguageOverride: '{currentOverride}'");
+                Debug.WriteLine($"System Display Languages: {string.Join(", ", systemLanguages)}");
+                Debug.WriteLine($"Application Languages: {string.Join(", ", appLanguages)}");
+                Debug.WriteLine("===========================");
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Error getting language debug info: {ex.Message}");
+            }
+        }
+    }
+
+    public class LanguageInfo
+    {
+        public string Code { get; set; }
+        public string NativeName { get; set; }
+
+        public override string ToString()
+        {
+            return string.IsNullOrEmpty(NativeName) ? Code : NativeName;
+        }
+    }
+}

src/Calculator/Views/Settings.xaml

@@ -109,6 +109,21 @@
                     </toolkit:SettingsExpander.Items>
                 </toolkit:SettingsExpander>
 
+                <toolkit:SettingsExpander x:Name="LanguageExpander"
+                                          Header="Language"
+                                          Description="Choose application language (requires restart for full effect)">
+                    <toolkit:SettingsExpander.HeaderIcon>
+                        <FontIcon Glyph="&#xF2B7;"/>
+                    </toolkit:SettingsExpander.HeaderIcon>
+                    <toolkit:SettingsExpander.Items>
+                        <toolkit:SettingsCard ContentAlignment="Left">
+                            <ComboBox x:Name="LanguageComboBox"
+                                      MinWidth="220"
+                                      SelectionChanged="OnLanguageSelectionChanged"/>
+                        </toolkit:SettingsCard>
+                    </toolkit:SettingsExpander.Items>
+                </toolkit:SettingsExpander>
+
                 <TextBlock x:Name="AboutGroupTitle"
                            Margin="0,30,0,4"
                            Style="{StaticResource BodyStrongTextBlockStyle}"

src/Calculator/Views/Settings.xaml.cs

@@ -13,6 +13,7 @@ using Windows.UI.Xaml;
 using Windows.UI.Xaml.Automation.Peers;
 using Windows.UI.Xaml.Automation.Provider;
 using Windows.UI.Xaml.Controls;
+using System.Collections.Generic;
 
 // The User Control item template is documented at https://go.microsoft.com/fwlink/?LinkId=234236
 
@@ -21,6 +22,7 @@ namespace CalculatorApp
     public sealed partial class Settings : UserControl
     {
         private const string BUILD_YEAR = "2025";
+        private bool _isSettingLanguageProgrammatically = false;
 
         public event Windows.UI.Xaml.RoutedEventHandler BackButtonClick;
 
@@ -48,6 +50,8 @@ namespace CalculatorApp
             AboutExpander.Description = copyrightText;
 
             InitializeContributeTextBlock();
+
+            InitializeLanguageSettings();
         }
 
         private void OnThemeSelectionChanged(object sender, SelectionChangedEventArgs e)
@@ -73,6 +77,27 @@ namespace CalculatorApp
             var currentTheme = ThemeHelper.RootTheme.ToString();
             (ThemeRadioButtons.Items.Cast<RadioButton>().FirstOrDefault(c => c?.Tag?.ToString() == currentTheme)).IsChecked = true;
 
+            // Initialize language selection to current preference
+            try
+            {
+                var items = LanguageComboBox.ItemsSource as List<CalculatorApp.Utils.LanguageInfo>;
+                var selected = CalculatorApp.Utils.LanguageHelper.SelectedLanguage;
+                if (items != null)
+                {
+                    foreach (var it in items)
+                    {
+                        if (it.Code == selected)
+                        {
+                            _isSettingLanguageProgrammatically = true;
+                            LanguageComboBox.SelectedItem = it;
+                            _isSettingLanguageProgrammatically = false;
+                            break;
+                        }
+                    }
+                }
+            }
+            catch { _isSettingLanguageProgrammatically = false; }
+
             SetDefaultFocus();
         }
 
@@ -139,6 +164,48 @@ namespace CalculatorApp
             ContributeRunAfterLink.Text = contributeTextAfterHyperlink;
         }
 
+        private void InitializeLanguageSettings()
+        {
+            try
+            {
+                var list = CalculatorApp.Utils.LanguageHelper.GetAvailableLanguages();
+                LanguageComboBox.ItemsSource = list;
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"Language init error: {ex.Message}");
+            }
+        }
+
+        private void OnLanguageSelectionChanged(object sender, SelectionChangedEventArgs e)
+        {
+            if (_isSettingLanguageProgrammatically)
+            {
+                return;
+            }
+
+            if (e.AddedItems.Count == 0)
+                return;
+
+            if (e.AddedItems[0] is CalculatorApp.Utils.LanguageInfo lang)
+            {
+                var current = CalculatorApp.Utils.LanguageHelper.SelectedLanguage;
+                if (string.Equals(current, lang.Code, StringComparison.OrdinalIgnoreCase))
+                {
+                    return; // no actual change
+                }
+
+                CalculatorApp.Utils.LanguageHelper.ApplyUserLanguageSelection(lang.Code);
+
+                // Inform user that full app restart may be required to update all resources
+                var title = AppResourceProvider.GetInstance().GetResourceString("LanguageChangeDialog/Title");
+                var message = AppResourceProvider.GetInstance().GetResourceString("LanguageChangeDialog/Message");
+                if (string.IsNullOrEmpty(title)) title = "Language Changed";
+                if (string.IsNullOrEmpty(message)) message = "Some UI may update after restart.";
+                _ = new ContentDialog { Title = title, Content = message, PrimaryButtonText = "OK" }.ShowAsync();
+            }
+        }
+
         private void System_BackRequested(object sender, BackRequestedEventArgs e)
         {
             if (!e.Handled && BackButton.IsEnabled)