diff --git a/SharepointToolbox.Tests/Services/SettingsServiceTests.cs b/SharepointToolbox.Tests/Services/SettingsServiceTests.cs
index 14a9ad3..99ceb09 100644
--- a/SharepointToolbox.Tests/Services/SettingsServiceTests.cs
+++ b/SharepointToolbox.Tests/Services/SettingsServiceTests.cs
@@ -1,7 +1,123 @@
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Infrastructure.Persistence;
+using SharepointToolbox.Services;
+
 namespace SharepointToolbox.Tests.Services;
 
-public class SettingsServiceTests
+[Trait("Category", "Unit")]
+public class SettingsServiceTests : IDisposable
 {
-    [Fact(Skip = "Wave 0 stub — implemented in plan 01-03")]
-    public void SaveAndLoad_RoundTrips_Settings() { }
+    private readonly string _tempFile;
+
+    public SettingsServiceTests()
+    {
+        _tempFile = Path.GetTempFileName();
+        File.Delete(_tempFile);
+    }
+
+    public void Dispose()
+    {
+        if (File.Exists(_tempFile)) File.Delete(_tempFile);
+        if (File.Exists(_tempFile + ".tmp")) File.Delete(_tempFile + ".tmp");
+    }
+
+    private SettingsRepository CreateRepository() => new(_tempFile);
+    private SettingsService CreateService() => new(CreateRepository());
+
+    [Fact]
+    public async Task LoadAsync_MissingFile_ReturnsDefaultSettings()
+    {
+        var repo = CreateRepository();
+
+        var settings = await repo.LoadAsync();
+
+        Assert.Equal(string.Empty, settings.DataFolder);
+        Assert.Equal("en", settings.Lang);
+    }
+
+    [Fact]
+    public async Task SaveAndLoad_RoundTrips_DataFolderAndLang()
+    {
+        var repo = CreateRepository();
+        var original = new AppSettings { DataFolder = @"C:\Exports", Lang = "fr" };
+
+        await repo.SaveAsync(original);
+        var loaded = await repo.LoadAsync();
+
+        Assert.Equal(@"C:\Exports", loaded.DataFolder);
+        Assert.Equal("fr", loaded.Lang);
+    }
+
+    [Fact]
+    public async Task SaveAsync_SerializedJson_UsesDataFolderAndLangKeys()
+    {
+        var repo = CreateRepository();
+        await repo.SaveAsync(new AppSettings { DataFolder = @"C:\Exports", Lang = "fr" });
+
+        var json = await File.ReadAllTextAsync(_tempFile);
+        using var doc = JsonDocument.Parse(json);
+
+        Assert.True(doc.RootElement.TryGetProperty("dataFolder", out _),
+            "JSON must contain 'dataFolder' key (camelCase for schema compatibility)");
+        Assert.True(doc.RootElement.TryGetProperty("lang", out _),
+            "JSON must contain 'lang' key (camelCase for schema compatibility)");
+    }
+
+    [Fact]
+    public async Task SaveAsync_UsesTmpFileThenMove()
+    {
+        var repo = CreateRepository();
+
+        // The .tmp file should not exist after a successful save
+        await repo.SaveAsync(new AppSettings { DataFolder = @"C:\Test", Lang = "en" });
+
+        Assert.False(File.Exists(_tempFile + ".tmp"),
+            "Temp file should have been moved/deleted after successful save");
+        Assert.True(File.Exists(_tempFile), "Settings file must exist after save");
+    }
+
+    [Fact]
+    public async Task SetLanguageAsync_PersistsLang()
+    {
+        var service = CreateService();
+
+        await service.SetLanguageAsync("fr");
+
+        var settings = await service.GetSettingsAsync();
+        Assert.Equal("fr", settings.Lang);
+    }
+
+    [Fact]
+    public async Task SetDataFolderAsync_PersistsPath()
+    {
+        var service = CreateService();
+
+        await service.SetDataFolderAsync(@"C:\Exports");
+
+        var settings = await service.GetSettingsAsync();
+        Assert.Equal(@"C:\Exports", settings.DataFolder);
+    }
+
+    [Fact]
+    public async Task SetDataFolderAsync_EmptyString_IsAllowed()
+    {
+        var service = CreateService();
+        await service.SetDataFolderAsync(@"C:\Exports");
+
+        await service.SetDataFolderAsync(string.Empty);
+
+        var settings = await service.GetSettingsAsync();
+        Assert.Equal(string.Empty, settings.DataFolder);
+    }
+
+    [Fact]
+    public async Task SetLanguageAsync_InvalidCode_ThrowsArgumentException()
+    {
+        var service = CreateService();
+
+        await Assert.ThrowsAsync<ArgumentException>(() => service.SetLanguageAsync("de"));
+    }
 }
diff --git a/SharepointToolbox/Core/Models/AppSettings.cs b/SharepointToolbox/Core/Models/AppSettings.cs
new file mode 100644
index 0000000..f0538a0
--- /dev/null
+++ b/SharepointToolbox/Core/Models/AppSettings.cs
@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Core.Models;
+
+public class AppSettings
+{
+    public string DataFolder { get; set; } = string.Empty;
+    public string Lang { get; set; } = "en";
+}
diff --git a/SharepointToolbox/Infrastructure/Persistence/SettingsRepository.cs b/SharepointToolbox/Infrastructure/Persistence/SettingsRepository.cs
new file mode 100644
index 0000000..f038c28
--- /dev/null
+++ b/SharepointToolbox/Infrastructure/Persistence/SettingsRepository.cs
@@ -0,0 +1,74 @@
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Infrastructure.Persistence;
+
+public class SettingsRepository
+{
+    private readonly string _filePath;
+    private readonly SemaphoreSlim _writeLock = new(1, 1);
+
+    public SettingsRepository(string filePath)
+    {
+        _filePath = filePath;
+    }
+
+    public async Task<AppSettings> LoadAsync()
+    {
+        if (!File.Exists(_filePath))
+            return new AppSettings();
+
+        string json;
+        try
+        {
+            json = await File.ReadAllTextAsync(_filePath, Encoding.UTF8);
+        }
+        catch (IOException ex)
+        {
+            throw new InvalidDataException($"Failed to read settings file: {_filePath}", ex);
+        }
+
+        try
+        {
+            var settings = JsonSerializer.Deserialize<AppSettings>(json,
+                new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+            return settings ?? new AppSettings();
+        }
+        catch (JsonException ex)
+        {
+            throw new InvalidDataException($"Settings file contains invalid JSON: {_filePath}", ex);
+        }
+    }
+
+    public async Task SaveAsync(AppSettings settings)
+    {
+        await _writeLock.WaitAsync();
+        try
+        {
+            var json = JsonSerializer.Serialize(settings,
+                new JsonSerializerOptions
+                {
+                    WriteIndented = true,
+                    PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+                });
+
+            var tmpPath = _filePath + ".tmp";
+            var dir = Path.GetDirectoryName(_filePath);
+            if (!string.IsNullOrEmpty(dir))
+                Directory.CreateDirectory(dir);
+
+            await File.WriteAllTextAsync(tmpPath, json, Encoding.UTF8);
+
+            // Validate round-trip before replacing
+            JsonDocument.Parse(await File.ReadAllTextAsync(tmpPath, Encoding.UTF8)).Dispose();
+
+            File.Move(tmpPath, _filePath, overwrite: true);
+        }
+        finally
+        {
+            _writeLock.Release();
+        }
+    }
+}
diff --git a/SharepointToolbox/Services/SettingsService.cs b/SharepointToolbox/Services/SettingsService.cs
new file mode 100644
index 0000000..5353d89
--- /dev/null
+++ b/SharepointToolbox/Services/SettingsService.cs
@@ -0,0 +1,39 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Infrastructure.Persistence;
+
+namespace SharepointToolbox.Services;
+
+public class SettingsService
+{
+    private readonly SettingsRepository _repository;
+
+    private static readonly HashSet<string> SupportedLanguages = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "en", "fr"
+    };
+
+    public SettingsService(SettingsRepository repository)
+    {
+        _repository = repository;
+    }
+
+    public Task<AppSettings> GetSettingsAsync()
+        => _repository.LoadAsync();
+
+    public async Task SetLanguageAsync(string cultureCode)
+    {
+        if (!SupportedLanguages.Contains(cultureCode))
+            throw new ArgumentException($"Unsupported language code '{cultureCode}'. Supported: en, fr.", nameof(cultureCode));
+
+        var settings = await _repository.LoadAsync();
+        settings.Lang = cultureCode;
+        await _repository.SaveAsync(settings);
+    }
+
+    public async Task SetDataFolderAsync(string path)
+    {
+        var settings = await _repository.LoadAsync();
+        settings.DataFolder = path;
+        await _repository.SaveAsync(settings);
+    }
+}