Localise the UI into Russian and English, switchable without a restart

Strings move into Localization/Strings.resx plus a Russian satellite. XAML uses
a {l:Loc Key} markup extension that yields a binding through the localizer's
indexer rather than a resolved string, so changing the language raises
PropertyChanged for the indexer and every caption in the app re-reads at once.
Resolving strings once at load would have been simpler and would have needed an
app restart to take effect.

Three places where this is more than a string swap:

Russian has three plural forms, so counted messages are assembled from .One /
.Few / .Many keys via Localizer.Plural instead of "{0} records" with an English
plural glued on. "1 запись", "3 записи", "7 записей".

Enum captions in pickers go through LocalizedOption<T>. A value converter would
resolve the caption once and never notice a language change; the wrapper keeps
identity on the enum value so the selection survives, while the label follows
the localizer. It needed a non-generic base for DataTemplate x:DataType, since
Avalonia 12 compiles bindings by default and cannot infer one for an open
generic.

Text originating in the domain would otherwise have stayed English under a
Russian UI — the screenshots showed exactly that. AvParser.Core still knows
nothing about languages: ParseError now carries a Code and Arguments, and the UI
translates Parse.Error.{Code} with a fallback to the English message. Parser
names work the same way (Parser.{id}.Name falling back to DisplayName), which
keeps "add a parser = one registration line" true — an untranslated parser shows
its own name rather than a missing-key marker.

Both .resx files are generated from one table so a key cannot exist in one and
be missing from the other, and the tests assert that, plus no blank translations
and identical {0} placeholder sets — a translation that drops a placeholder
throws at runtime rather than merely reading oddly. A headless test switches
language on a live shell and asserts the rendered text changes without the tree
being rebuilt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/AvParser.UI.HeadlessTests/Fakes.cs

@@ -14,9 +14,9 @@ namespace AvParser.UI.HeadlessTests;
 /// project: these are a few lines of trivial stand-ins, and a shared test-kit assembly would have
 /// to opt out of the <c>tests/</c> conventions (self-executing xUnit exe) to build at all.
 /// </remarks>
-internal sealed class FakePage(string title, string iconKey = "IconHome") : PageViewModel
+internal sealed class FakePage(string titleKey, string iconKey = "IconHome") : PageViewModel
 {
-    public override string Title { get; } = title;
+    public override string TitleKey { get; } = titleKey;
 
     public override string IconKey { get; } = iconKey;
 }

tests/AvParser.UI.HeadlessTests/LocalizationViewTests.cs

@@ -0,0 +1,90 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+using AvParser.Core.Settings;
+using AvParser.UI.Localization;
+using AvParser.UI.Navigation;
+using AvParser.UI.ViewModels;
+using AvParser.UI.Views;
+using ReactiveUI.Primitives.Concurrency;
+
+namespace AvParser.UI.HeadlessTests;
+
+public sealed class LocalizationViewTests : IDisposable
+{
+    /// <summary>Leaves the localizer where the rest of the assembly expects it.</summary>
+    public void Dispose() => Localizer.Instance.SetLanguage(AppLanguage.English);
+
+    private static (ShellView View, ShellViewModel ViewModel) ShowShell()
+    {
+        var navigation = new NavigationService([new FakePage("Page.Dashboard"), new FakePage("Page.Parse")]);
+        var viewModel = new ShellViewModel(navigation, new FakeThemeService(), ImmediateSequencer.Instance);
+        var view = new ShellView { DataContext = viewModel };
+
+        new Window
+        {
+            Width = 1400,
+            Height = 800,
+            Content = view,
+        }.Show();
+
+        Dispatcher.UIThread.RunJobs();
+        return (view, viewModel);
+    }
+
+    private static IReadOnlyList<string> RailLabels(Visual view) =>
+        [
+            .. view.GetVisualDescendants()
+                .OfType<TextBlock>()
+                .Where(block => block.Classes.Contains("navLabel"))
+                .Select(block => block.Text ?? string.Empty),
+        ];
+
+    [AvaloniaFact]
+    public void Rendered_text_follows_the_language_without_rebuilding_the_view()
+    {
+        Localizer.Instance.SetLanguage(AppLanguage.English);
+        var (view, _) = ShowShell();
+
+        var english = RailLabels(view);
+        english.ShouldBe(["Dashboard", "Parse"]);
+
+        Localizer.Instance.SetLanguage(AppLanguage.Russian);
+        Dispatcher.UIThread.RunJobs();
+
+        // The same controls, not a rebuilt tree: this is the whole point of binding through the
+        // localizer's indexer rather than resolving strings once at load.
+        RailLabels(view).ShouldBe(["Обзор", "Разбор"]);
+    }
+
+    [AvaloniaFact]
+    public void The_title_bar_follows_the_language_too()
+    {
+        Localizer.Instance.SetLanguage(AppLanguage.English);
+        var (view, viewModel) = ShowShell();
+
+        var title = view.GetVisualDescendants().OfType<TextBlock>().Single(block => block.Name == "ShellTitle");
+        title.Text.ShouldBe("Dashboard");
+
+        Localizer.Instance.SetLanguage(AppLanguage.Russian);
+        Dispatcher.UIThread.RunJobs();
+
+        viewModel.Title.ShouldBe("Обзор");
+        title.Text.ShouldBe("Обзор");
+    }
+
+    [AvaloniaFact]
+    public void Loc_markup_resolves_a_static_caption()
+    {
+        Localizer.Instance.SetLanguage(AppLanguage.Russian);
+        var (view, _) = ShowShell();
+
+        // Tooltips come from {l:Loc} rather than from a view model, so they exercise the markup
+        // extension itself.
+        var toggle = view.GetVisualDescendants().OfType<Button>().Single(button => button.Name == "PaneToggle");
+
+        ToolTip.GetTip(toggle).ShouldBe("Показать или скрыть навигацию");
+    }
+}

tests/AvParser.UI.HeadlessTests/ShellViewTests.cs

@@ -5,6 +5,7 @@ using Avalonia.Styling;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 using AvParser.Core.Settings;
+using AvParser.UI.Localization;
 using AvParser.UI.Navigation;
 using AvParser.UI.Responsive;
 using AvParser.UI.Services;
@@ -27,7 +28,10 @@ public class ShellViewTests
     /// </remarks>
     private static (ShellView View, ShellViewModel ViewModel, Window Window) ShowShell(double width)
     {
-        var navigation = new NavigationService([new FakePage("First"), new FakePage("Second", "IconDocument")]);
+        var navigation = new NavigationService([
+            new FakePage("Page.Dashboard"),
+            new FakePage("Page.Parse", "IconDocument"),
+        ]);
 
         // ImmediateSequencer, not the real main-thread one: derived properties then settle within
         // the same layout pass, so a single RunJobs() is enough for the bindings to catch up.
@@ -108,7 +112,7 @@ public class ShellViewTests
 
         var host = view.GetVisualDescendants().OfType<TransitioningContentControl>().Single();
         host.Content.ShouldBeSameAs(second);
-        viewModel.Title.ShouldBe("Second");
+        viewModel.Title.ShouldBe(Localizer.Instance["Page.Parse"]);
     }
 
     /// <summary>