Add multilingual support with language selector

Introduced a `LanguageControls` component for switching between
languages (`de-DE`, `en-US`, `fr-FR`) and integrated a new
`LocalizationService` to manage language preferences. Updated
`LocalizationController` to handle language persistence via
cookies and support multiple cultures. Enhanced the footer
layout to include the language selector alongside theme
controls.

Added `flag-icons` library for displaying country flags and
configured request localization in `Program.cs`. Styled the
language selector dropdown and improved sender page visuals
with updated CSS. Defaulted language handling to `de-DE` for
fallback scenarios. Cleaned up redundant code in
`LocalizationController`.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Layout/LanguageControls.razor

@@ -0,0 +1,84 @@
+@using EnvelopeGenerator.Server.Client.Services
+@rendermode InteractiveWebAssembly
+
+@inject LocalizationService LocalizationService
+@inject NavigationManager Navigation
+
+<div class="sf-lang-combo">
+    <button type="button"
+            class="sf-lang-trigger"
+            @onclick="ToggleMenu"
+            aria-label="Language"
+            aria-expanded="@_isOpen">
+        <span class="fi @_selected.FlagClass"></span>
+    </button>
+
+    @if (_isOpen)
+    {
+        <button type="button"
+                class="sf-lang-backdrop"
+                aria-label="Close language menu"
+                @onclick="CloseMenu">
+        </button>
+
+        <div class="sf-lang-menu" role="listbox" aria-label="Languages">
+            @foreach (var option in _options)
+            {
+                <button type="button"
+                        class="sf-lang-option @(_selected.Code == option.Code ? "sf-lang-option--active" : string.Empty)"
+                        role="option"
+                        aria-selected="@(_selected.Code == option.Code)"
+                        @onmousedown:preventDefault="true"
+                        @onclick="() => SelectLanguageAsync(option)">
+                    <span class="fi @option.FlagClass"></span>
+                    <span>@option.ShortName</span>
+                </button>
+            }
+        </div>
+    }
+</div>
+
+@code {
+    private readonly IReadOnlyList<LanguageOption> _options =
+    [
+        new("de-DE", "fi-de", "DE"),
+        new("en-US", "fi-us", "EN"),
+        new("fr-FR", "fi-fr", "FR")
+    ];
+
+    private LanguageOption _selected = new("de-DE", "fi-de", "DE");
+    private bool _isOpen;
+
+    protected override async Task OnInitializedAsync()
+    {
+        var currentLanguage = await LocalizationService.GetLanguageAsync();
+        _selected = _options.FirstOrDefault(x => x.Code.Equals(currentLanguage, StringComparison.OrdinalIgnoreCase))
+            ?? _options[0];
+    }
+
+    private void ToggleMenu()
+    {
+        _isOpen = !_isOpen;
+    }
+
+    private void CloseMenu()
+    {
+        _isOpen = false;
+    }
+
+    private async Task SelectLanguageAsync(LanguageOption option)
+    {
+        if (_selected.Code.Equals(option.Code, StringComparison.OrdinalIgnoreCase))
+        {
+            _isOpen = false;
+            return;
+        }
+
+        _selected = option;
+        _isOpen = false;
+        await LocalizationService.SetLanguageAsync(option.Code);
+        Navigation.NavigateTo(Navigation.Uri, forceLoad: true);
+    }
+
+    private sealed record LanguageOption(string Code, string FlagClass, string ShortName);
+}

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Layout/MainLayout.razor

@@ -21,7 +21,10 @@
             <a href="docs/privacy-policy.de-DE.html" target="_blank" rel="noopener">Datenschutz</a>
         </div>
 
-        <ThemeControls />
+        <div class="receiver-footer__right">
+            <LanguageControls />
+            <ThemeControls />
+        </div>
     </footer>
 </div>
 

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Program.cs

@@ -33,6 +33,7 @@ builder.Services.AddScoped<SignatureService>();
 builder.Services.AddScoped<SignatureCacheService>();
 builder.Services.AddSingleton<AppVersionService>();
 builder.Services.AddScoped<EnvelopeService>();
+builder.Services.AddScoped<LocalizationService>();
 
 // UI Preferences & Theme
 builder.Services.AddScoped<UserPreferencesService>();

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/LocalizationService.cs

@@ -0,0 +1,30 @@
+using System.Net;
+
+namespace EnvelopeGenerator.Server.Client.Services;
+
+public class LocalizationService(IHttpClientFactory httpClientFactory)
+{
+    private HttpClient CreateDefaultClient() => httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+
+    public async Task<string> GetLanguageAsync(CancellationToken cancel = default)
+    {
+        using var http = CreateDefaultClient();
+        var response = await http.GetAsync("/api/Localization/lang", cancel);
+
+        if (response.StatusCode == HttpStatusCode.NotFound)
+            return "de-DE";
+
+        if (!response.IsSuccessStatusCode)
+            return "de-DE";
+
+        var language = await response.Content.ReadAsStringAsync(cancel);
+        return string.IsNullOrWhiteSpace(language) ? "de-DE" : language.Trim('"');
+    }
+
+    public async Task SetLanguageAsync(string language, CancellationToken cancel = default)
+    {
+        using var http = CreateDefaultClient();
+        var response = await http.PostAsync($"/api/Localization/lang?language={Uri.EscapeDataString(language)}", null, cancel);
+        response.EnsureSuccessStatusCode();
+    }
+}

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/App.razor

@@ -6,6 +6,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <base href="/" />
     <link rel="stylesheet" href="css/bootstrap/bootstrap.min.css" />
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flag-icons@7.2.3/css/flag-icons.min.css" />
     <link rel="stylesheet" href="css/app.css" />
     <link rel="stylesheet" href="css/envelope-viewer.css" />
     <link rel="stylesheet" href="EnvelopeGenerator.Server.styles.css" />

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderPage.razor

@@ -527,6 +527,7 @@
             </div>
         }
     </div>
+
 </div>
 
 @* ── PDF Vorschau Popup ────────────────────────────────────────────────── *@

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Controllers/LocalizationController.cs

@@ -4,7 +4,6 @@ using Microsoft.AspNetCore.Localization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.Extensions.Caching.Memory;
 using Microsoft.Extensions.Localization;
-using EnvelopeGenerator.Application.Resources;
 
 namespace EnvelopeGenerator.Server.Controllers;
 
@@ -20,7 +19,6 @@ public class LocalizationController : ControllerBase
 
     private readonly ILogger<LocalizationController> _logger;
     private readonly IStringLocalizer<Resource> _mLocalizer;
-    private readonly IStringLocalizer<Resource> _localizer;
     private readonly IMemoryCache _cache;
 
     /// <summary>
@@ -32,14 +30,12 @@ public class LocalizationController : ControllerBase
     /// <param name="_modelLocalizer">Lokalisierungsdienst für Modelle.</param>
     public LocalizationController(
         ILogger<LocalizationController> logger,
-        IStringLocalizer<Resource> localizer,
         IMemoryCache memoryCache,
-        IStringLocalizer<Resource> _modelLocalizer)
+        IStringLocalizer<Resource> modelLocalizer)
     {
         _logger = logger;
-        _localizer = localizer;
         _cache = memoryCache;
-        _mLocalizer = _modelLocalizer;
+        _mLocalizer = modelLocalizer;
     }
 
     /// <summary>

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -337,6 +337,7 @@ try
     builder.Services.AddScoped<EnvelopeGenerator.Server.Client.Services.UserPreferencesService>();
     builder.Services.AddScoped<EnvelopeGenerator.Server.Client.Services.ThemeService>();
     builder.Services.AddScoped<EnvelopeGenerator.Server.Client.Services.CustomImagesService>();
+    builder.Services.AddScoped<EnvelopeGenerator.Server.Client.Services.LocalizationService>();
 
     // Business Services (Server specific)
     builder.Services.AddScoped<DocumentService>();
@@ -410,9 +411,14 @@ try
     // Set CORS policy
     app.UseCors("AllowSpecificOriginsPolicy");
 
-    // Localization (fixed to German for now; persisted via cookie)
+    // Localization (default German; can be switched via cookie)
     var defaultCulture = new CultureInfo("de-DE");
-    IList<CultureInfo> list = [defaultCulture];
+    IList<CultureInfo> list =
+    [
+        defaultCulture,
+        new CultureInfo("en-US"),
+        new CultureInfo("fr-FR")
+    ];
     var requestLocalizationOptions = new RequestLocalizationOptions
     {
         DefaultRequestCulture = new RequestCulture(defaultCulture),
@@ -420,6 +426,7 @@ try
         SupportedUICultures = list,
         RequestCultureProviders = [new CookieRequestCultureProvider()]
     };
+    requestLocalizationOptions.ApplyCurrentCultureToResponseHeaders = true;
     app.UseRequestLocalization(requestLocalizationOptions);
 
     app.UseHttpsRedirection();

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/app.css

@@ -404,6 +404,89 @@ article {
     opacity: 0.4;
 }
 
+.receiver-footer__right {
+    display: inline-flex;
+    align-items: center;
+    gap: 12px;
+}
+
+.sf-lang-combo {
+    position: relative;
+    display: inline-flex;
+    align-items: center;
+}
+
+.sf-lang-trigger {
+    width: 28px;
+    height: 20px;
+    border: 1px solid rgba(255, 255, 255, 0.28);
+    border-radius: 5px;
+    background: rgba(255, 255, 255, 0.08);
+    padding: 0;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    cursor: pointer;
+}
+
+.sf-lang-trigger:hover {
+    background: rgba(255, 255, 255, 0.16);
+}
+
+.sf-lang-menu {
+    position: absolute;
+    right: 0;
+    bottom: 28px;
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+    min-width: 64px;
+    padding: 6px;
+    border-radius: 8px;
+    border: 1px solid rgba(255, 255, 255, 0.18);
+    background: rgba(29, 38, 55, 0.98);
+    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.3);
+    z-index: 110;
+}
+
+.sf-lang-backdrop {
+    position: fixed;
+    inset: 0;
+    border: 0;
+    background: transparent;
+    padding: 0;
+    margin: 0;
+    cursor: default;
+    z-index: 105;
+}
+
+.sf-lang-option {
+    display: inline-flex;
+    align-items: center;
+    gap: 6px;
+    border: 0;
+    border-radius: 6px;
+    background: transparent;
+    color: rgba(255, 255, 255, 0.88);
+    font-size: 0.68rem;
+    font-weight: 600;
+    padding: 4px 6px;
+    cursor: pointer;
+}
+
+.sf-lang-option:hover,
+.sf-lang-option--active {
+    background: rgba(255, 255, 255, 0.14);
+}
+
+.sf-lang-trigger .fi,
+.sf-lang-option .fi {
+    width: 16px;
+    height: 12px;
+    border-radius: 2px;
+    box-shadow: 0 0 0 1px rgba(255, 255, 255, 0.25) inset;
+}
+
 /* ── Footer: Theme Slider ────────────────────────────────────────────────── */
 /* 🌙 blazing-dark ──◇──·──◇── purple ☀️                                     */
 /* Left group: copyright | datenschutz  →  Right group: slider               */

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/sender-page.css

@@ -238,6 +238,7 @@
     overflow: auto;
 }
 
+
 .sender-grid-container {
     background: rgba(255, 255, 255, 0.98);
     backdrop-filter: blur(20px);
@@ -690,6 +691,7 @@
     box-shadow: 0 25px 50px -12px rgba(0, 0, 0, 0.5), 0 0 0 1px rgba(255, 255, 255, 0.06);
 }
 
+
 [data-sf-theme="blazing-dark"] .sender-grid-container::before {
     background: linear-gradient(90deg, #6366f1 0%, #4338ca 100%);
 }