diff --git a/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs b/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs
index 3c06d46..63eed90 100644
--- a/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs
+++ b/YKanBan.Tests/Storage/AppData/AppConfigStoreTests.cs
@@ -158,4 +158,148 @@ public class AppConfigStoreTests
         Assert.AreEqual(config.ColumnWidth.Preset, reloaded.ColumnWidth.Preset);
         Assert.AreEqual(config.Confirmations.DiscardEdit, reloaded.Confirmations.DiscardEdit);
     }
+
+    [TestMethod]
+    public void LoadNonObjectRootBacksUpAndReturnsDefaults()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, "[1, 2]");
+        var store = new AppConfigStore(path);
+
+        Assert.AreEqual(AppConfig.DefaultLanguage, store.Load().Language);
+        Assert.AreEqual("[1, 2]", File.ReadAllText(path + ".bak"));
+    }
+
+    [TestMethod]
+    [DataRow("\"theme\": \"sepia\"")]
+    [DataRow("\"theme\": 1")]
+    [DataRow("\"theme\": {}")]
+    [DataRow("\"language\": \"fr\"")]
+    [DataRow("\"language\": \"zh-hans\"")]
+    [DataRow("\"language\": 5")]
+    [DataRow("\"sort\": { \"card\": \"created-at\" }")]
+    [DataRow("\"sort\": 7")]
+    [DataRow("\"column-width\": { \"preset\": \"huge\", \"custom-pixels\": 159 }")]
+    [DataRow("\"column-width\": { \"custom-pixels\": 721 }")]
+    [DataRow("\"column-width\": { \"custom-pixels\": \"wide\" }")]
+    [DataRow("\"confirmations\": { \"delete-card\": \"nope\" }")]
+    [DataRow("\"version\": \"one\"")]
+    public void InvalidFieldFallsBackAloneWithoutBackup(string invalidMember)
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+
+        // The valid fields are non-defaults, so any whole-file fallback would show.
+        File.WriteAllText(path, $$"""
+            {
+              "theme": "dark",
+              "confirmations": { "discard-edit": false },
+              {{invalidMember}}
+            }
+            """);
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.IsFalse(File.Exists(path + ".bak"));
+        AppConfig defaults = new();
+        Assert.AreEqual(invalidMember.Contains("theme") ? defaults.Theme : ThemeOption.Dark, config.Theme);
+        if (!invalidMember.Contains("confirmations"))
+        {
+            Assert.IsFalse(config.Confirmations.DiscardEdit);
+        }
+        Assert.AreEqual(defaults.Language, config.Language);
+        Assert.AreEqual(defaults.Sort.Card, config.Sort.Card);
+        Assert.AreEqual(defaults.ColumnWidth.Preset, config.ColumnWidth.Preset);
+        Assert.AreEqual(defaults.ColumnWidth.CustomPixels, config.ColumnWidth.CustomPixels);
+        Assert.AreEqual(defaults.Confirmations.DeleteCard, config.Confirmations.DeleteCard);
+        Assert.AreEqual(defaults.Version, config.Version);
+    }
+
+    [TestMethod]
+    public void InvalidNestedFieldKeepsItsSiblings()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        File.WriteAllText(path, """{ "column-width": { "preset": "bogus", "custom-pixels": 500 } }""");
+
+        AppConfig config = new AppConfigStore(path).Load();
+
+        Assert.AreEqual(ColumnWidthPreset.Standard, config.ColumnWidth.Preset);
+        Assert.AreEqual(500, config.ColumnWidth.CustomPixels);
+    }
+
+    [TestMethod]
+    public void ColumnWidthBoundsAreInclusive()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+
+        File.WriteAllText(path, """{ "column-width": { "custom-pixels": 160 } }""");
+        Assert.AreEqual(160, new AppConfigStore(path).Load().ColumnWidth.CustomPixels);
+
+        File.WriteAllText(path, """{ "column-width": { "custom-pixels": 720 } }""");
+        Assert.AreEqual(720, new AppConfigStore(path).Load().ColumnWidth.CustomPixels);
+    }
+
+    [TestMethod]
+    public void UnchangedConfigurationIsNotWritten()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.IsFalse(File.Exists(path));
+    }
+
+    [TestMethod]
+    public void FieldFallbackAloneDoesNotRewriteTheFile()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        const string original = """{ "theme": "sepia" }""";
+        File.WriteAllText(path, original);
+        var store = new AppConfigStore(path);
+
+        AppConfig config = store.Load();
+
+        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.AreEqual(original, File.ReadAllText(path));
+    }
+
+    [TestMethod]
+    public void ChangedConfigurationIsWrittenOnce()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        var store = new AppConfigStore(path);
+        AppConfig config = store.Load();
+
+        config.Sort.Card = CardSortOption.UpdatedAt;
+
+        Assert.IsTrue(store.SaveIfChanged(config));
+        Assert.AreEqual(CardSortOption.UpdatedAt, new AppConfigStore(path).Load().Sort.Card);
+
+        // Saved state is the new baseline.
+        Assert.IsFalse(store.SaveIfChanged(config));
+    }
+
+    [TestMethod]
+    public void ChangeRevertedWithinSessionIsNotWritten()
+    {
+        using var directory = new TempDirectory();
+        string path = Path.Combine(directory.FullPath, "app.json");
+        var store = new AppConfigStore(path);
+        AppConfig config = store.Load();
+
+        config.Theme = ThemeOption.Dark;
+        config.Theme = ThemeOption.FollowSystem;
+
+        Assert.IsFalse(store.SaveIfChanged(config));
+        Assert.IsFalse(File.Exists(path));
+    }
 }
diff --git a/YKanBan/Services/AppServices.cs b/YKanBan/Services/AppServices.cs
index ac6b593..37cb201 100644
--- a/YKanBan/Services/AppServices.cs
+++ b/YKanBan/Services/AppServices.cs
@@ -6,7 +6,7 @@ namespace YKanBan.Services;
 /// <summary>
 /// Composition root for the whole process: owns the loaded configuration and
 /// the single open workspace session. The configuration is persisted once at
-/// shutdown; the session (lock plus connection) lives until disposal.
+/// shutdown, and only when this session changed it; the session (lock plus connection) lives until disposal.
 /// </summary>
 public sealed class AppServices : IDisposable
 {
@@ -49,9 +49,9 @@ public sealed class AppServices : IDisposable
     }
 
     /// <summary>
-    /// Persists the configuration; called once at shutdown.
+    /// Persists the configuration when this session changed a setting; called once at shutdown.
     /// </summary>
-    public void SaveConfiguration() => ConfigStore.Save(Config);
+    public void SaveConfiguration() => ConfigStore.SaveIfChanged(Config);
 
     /// <summary>
     /// Releases the workspace session, if any.
diff --git a/YKanBan/Storage/AppData/AppConfig.cs b/YKanBan/Storage/AppData/AppConfig.cs
index 65beb6a..ec52fc4 100644
--- a/YKanBan/Storage/AppData/AppConfig.cs
+++ b/YKanBan/Storage/AppData/AppConfig.cs
@@ -141,6 +141,26 @@ public sealed class AppConfig
     /// </summary>
     public const string DefaultLanguage = "en";
 
+    /// <summary>
+    /// Simplified Chinese UI language code.
+    /// </summary>
+    public const string SimplifiedChineseLanguage = "zh-Hans";
+
+    /// <summary>
+    /// Gets the supported UI language codes; any other stored value falls back to <see cref="DefaultLanguage"/>.
+    /// </summary>
+    public static IReadOnlyList<string> SupportedLanguages { get; } = [DefaultLanguage, SimplifiedChineseLanguage];
+
+    /// <summary>
+    /// Smallest accepted custom column width in pixels.
+    /// </summary>
+    public const int MinColumnPixels = 160;
+
+    /// <summary>
+    /// Largest accepted custom column width in pixels.
+    /// </summary>
+    public const int MaxColumnPixels = 720;
+
     /// <summary>
     /// Gets or sets the configuration format version of this file.
     /// </summary>
diff --git a/YKanBan/Storage/AppData/AppConfigStore.cs b/YKanBan/Storage/AppData/AppConfigStore.cs
index 97e10ab..d9a18c4 100644
--- a/YKanBan/Storage/AppData/AppConfigStore.cs
+++ b/YKanBan/Storage/AppData/AppConfigStore.cs
@@ -1,13 +1,17 @@
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
+using Newtonsoft.Json.Linq;
 
 namespace YKanBan.Storage.AppData;
 
 /// <summary>
 /// Loads and saves app.json with graceful degradation: a missing file yields
-/// defaults; a corrupt file is backed up to app.json.bak and then defaults are
-/// used. Saving writes the file directly (no atomic replace) — losing the last
-/// session's settings to a crash is an accepted trade-off.
+/// defaults; a file that is not a JSON object is backed up to app.json.bak and
+/// then defaults are used; an individual invalid field (unknown enum value or
+/// language, out-of-range width, wrong type) falls back to its own default while
+/// the other fields are kept. Saving happens only when the configuration differs
+/// from what was loaded, and writes the file directly (no atomic replace) —
+/// losing the last session's settings to a crash is an accepted trade-off.
 /// </summary>
 public sealed class AppConfigStore
 {
@@ -16,10 +20,24 @@ public sealed class AppConfigStore
         Formatting = Formatting.Indented,
         NullValueHandling = NullValueHandling.Ignore,
 
-        // Enums round-trip through the kebab-case strings declared via [EnumMember].
-        Converters = { new StringEnumConverter() },
+        // Enums round-trip through the kebab-case strings declared via [EnumMember];
+        // numeric enum values are rejected like any other unknown value.
+        Converters = { new StringEnumConverter { AllowIntegerValues = false } },
     };
 
+    private static readonly JsonSerializer FieldTolerantSerializer = JsonSerializer.Create(new JsonSerializerSettings
+    {
+        NullValueHandling = NullValueHandling.Ignore,
+        Converters = { new StringEnumConverter { AllowIntegerValues = false } },
+
+        // The JSON text is already known to be well formed here, so every error is a
+        // single member that fails to convert: skip it and keep that member's default.
+        Error = (_, args) => args.ErrorContext.Handled = true,
+    });
+
+    /// <summary>The serialized form of the configuration as last loaded or saved.</summary>
+    private string _persistedSnapshot = Serialize(new AppConfig());
+
     /// <summary>
     /// Initializes a store bound to a specific app.json path.
     /// </summary>
@@ -32,12 +50,57 @@ public sealed class AppConfigStore
     public string FilePath { get; }
 
     /// <summary>
-    /// Loads the configuration. A missing file yields defaults; a corrupt file
-    /// is backed up to app.json.bak and then defaults are used.
+    /// Loads the configuration and records it as the persisted baseline for
+    /// <see cref="SaveIfChanged"/>. A missing file yields defaults; a file that
+    /// is not a JSON object is backed up to app.json.bak and then defaults are
+    /// used; invalid fields fall back to their defaults individually.
     /// </summary>
     /// <returns>The loaded configuration, or defaults when none could be read.</returns>
     /// <exception cref="IOException">The file exists but cannot be read.</exception>
     public AppConfig Load()
+    {
+        AppConfig config = Read();
+
+        // The baseline is the effective configuration, so field fallbacks alone never trigger a write.
+        _persistedSnapshot = Serialize(config);
+        return config;
+    }
+
+    /// <summary>
+    /// Writes the configuration only when it differs from the loaded or last
+    /// saved one, i.e. when this session changed a setting.
+    /// </summary>
+    /// <param name="config">The live configuration.</param>
+    /// <returns><see langword="true"/> when the file was written.</returns>
+    public bool SaveIfChanged(AppConfig config)
+    {
+        string json = Serialize(config);
+        if (json == _persistedSnapshot)
+        {
+            return false;
+        }
+
+        Write(json);
+        _persistedSnapshot = json;
+        return true;
+    }
+
+    /// <summary>
+    /// Writes the configuration directly to disk (no atomic replace), unconditionally.
+    /// </summary>
+    /// <param name="config">The configuration to serialize.</param>
+    public void Save(AppConfig config)
+    {
+        string json = Serialize(config);
+        Write(json);
+        _persistedSnapshot = json;
+    }
+
+    /// <summary>
+    /// Reads and sanitizes the file.
+    /// </summary>
+    /// <returns>The effective configuration.</returns>
+    private AppConfig Read()
     {
         // Missing file is a normal first-run state: run with defaults.
         if (!File.Exists(FilePath))
@@ -46,42 +109,98 @@ public sealed class AppConfigStore
         }
 
         string json = File.ReadAllText(FilePath);
+        JToken root;
         try
         {
-            // A JSON null literal deserializes to null; treat it like defaults.
-            AppConfig? config = JsonConvert.DeserializeObject<AppConfig>(json, SerializerSettings);
-            if (config is null)
-            {
-                return new AppConfig();
-            }
-
-            // Older or hand-edited files may miss whole sections; each falls back to its defaults.
-            config.Sort ??= new SortSettings();
-            config.ColumnWidth ??= new ColumnWidthSettings();
-            config.Confirmations ??= new ConfirmationSettings();
-            return config;
+            root = JToken.Parse(json);
         }
         catch (JsonException)
         {
-            // Corrupt: keep a copy for inspection, then continue with defaults.
-            File.Copy(FilePath, FilePath + ".bak", overwrite: true);
+            return BackUpAndUseDefaults();
+        }
+
+        // A JSON null literal carries no settings at all; treat it like a missing file.
+        if (root.Type == JTokenType.Null)
+        {
             return new AppConfig();
         }
+
+        // Well-formed JSON of the wrong shape has no field to keep: handle it like corrupt text.
+        if (root is not JObject rootObject)
+        {
+            return BackUpAndUseDefaults();
+        }
+
+        AppConfig config = rootObject.ToObject<AppConfig>(FieldTolerantSerializer) ?? new AppConfig();
+        Sanitize(config);
+        return config;
+    }
+
+    /// <summary>
+    /// Keeps a copy of an unusable file for inspection, then continues with defaults.
+    /// </summary>
+    /// <returns>The default configuration.</returns>
+    private AppConfig BackUpAndUseDefaults()
+    {
+        File.Copy(FilePath, FilePath + ".bak", overwrite: true);
+        return new AppConfig();
+    }
+
+    /// <summary>
+    /// Replaces every value that converted but is outside its domain with the field default.
+    /// </summary>
+    /// <param name="config">The freshly deserialized configuration.</param>
+    private static void Sanitize(AppConfig config)
+    {
+        var defaults = new AppConfig();
+
+        // Older or hand-edited files may miss whole sections; each falls back to its defaults.
+        config.Sort ??= new SortSettings();
+        config.ColumnWidth ??= new ColumnWidthSettings();
+        config.Confirmations ??= new ConfirmationSettings();
+
+        if (config.Language is null || !AppConfig.SupportedLanguages.Contains(config.Language, StringComparer.Ordinal))
+        {
+            config.Language = defaults.Language;
+        }
+        if (!Enum.IsDefined(config.Theme))
+        {
+            config.Theme = defaults.Theme;
+        }
+        if (!Enum.IsDefined(config.Sort.Card))
+        {
+            config.Sort.Card = defaults.Sort.Card;
+        }
+        if (!Enum.IsDefined(config.ColumnWidth.Preset))
+        {
+            config.ColumnWidth.Preset = defaults.ColumnWidth.Preset;
+        }
+        if (config.ColumnWidth.CustomPixels is < AppConfig.MinColumnPixels or > AppConfig.MaxColumnPixels)
+        {
+            config.ColumnWidth.CustomPixels = defaults.ColumnWidth.CustomPixels;
+        }
     }
 
     /// <summary>
-    /// Writes the configuration directly to disk (no atomic replace).
+    /// Writes the serialized configuration, creating the parent folder on demand.
     /// </summary>
-    /// <param name="config">The configuration to serialize.</param>
-    public void Save(AppConfig config)
+    /// <param name="json">The serialized configuration.</param>
+    private void Write(string json)
     {
-        // Create the parent folder on demand; normally it already exists.
+        // Normally the folder already exists.
         string? directory = Path.GetDirectoryName(FilePath);
         if (!string.IsNullOrEmpty(directory))
         {
             Directory.CreateDirectory(directory);
         }
 
-        File.WriteAllText(FilePath, JsonConvert.SerializeObject(config, SerializerSettings));
+        File.WriteAllText(FilePath, json);
     }
+
+    /// <summary>
+    /// Serializes a configuration in the on-disk format.
+    /// </summary>
+    /// <param name="config">The configuration.</param>
+    /// <returns>The JSON text.</returns>
+    private static string Serialize(AppConfig config) => JsonConvert.SerializeObject(config, SerializerSettings);
 }
diff --git a/YKanBan/ViewModels/Dialogs/SettingsDialogViewModel.cs b/YKanBan/ViewModels/Dialogs/SettingsDialogViewModel.cs
index cc83055..94f33fd 100644
--- a/YKanBan/ViewModels/Dialogs/SettingsDialogViewModel.cs
+++ b/YKanBan/ViewModels/Dialogs/SettingsDialogViewModel.cs
@@ -26,7 +26,7 @@ public sealed partial class SettingsDialogViewModel : ViewModelBase
         Languages =
         [
             new OptionItem(AppConfig.DefaultLanguage, Resources.Settings_LanguageEnglish),
-            new OptionItem("zh-Hans", Resources.Settings_LanguageChinese),
+            new OptionItem(AppConfig.SimplifiedChineseLanguage, Resources.Settings_LanguageChinese),
         ];
         selectedLanguage = Languages.FirstOrDefault(option => (string)option.Value == _config.Language)
             ?? Languages[0];