Feat: Phase 2 i18n Core API - LocalizedString View and Language Management

- Add LocalizedString View component for displaying localized strings
- Add Text(localized:) convenience initializer for Text views
- Add AppState.setLanguage(_:) method for runtime language switching
- Add AppState.currentLanguage computed property
- Integrate LocalizationService into RenderLoop environment
- Remove duplicate LocalizedString function (View takes precedence)
- All 1069 tests pass

Core i18n functionality complete. Ready for Phase 3 (String Replacement)

Sources/TUIkit/App/RenderLoop.swift

@@ -351,6 +351,7 @@ extension RenderLoop {
         environment.keyEventDispatcher = tuiContext.keyEventDispatcher
         environment.renderCache = tuiContext.renderCache
         environment.preferenceStorage = tuiContext.preferences
+        environment.localizationService = LocalizationService.shared
 
         return environment
     }

Sources/TUIkit/Localization/LocalizationExtensions.swift

@@ -0,0 +1,52 @@
+//  🖥️ TUIKit — Terminal UI Kit for Swift
+//  LocalizationExtensions.swift
+//
+//  Created by LAYERED.work
+//  License: MIT
+
+// MARK: - Text Localization Convenience
+
+extension Text {
+    /// Creates a text view with a localized string using a dot-notation key.
+    ///
+    /// # Example
+    ///
+    /// ```swift
+    /// Text(localized: "button.ok")
+    /// ```
+    ///
+    /// This is an alternative to `LocalizedString("button.ok")` for cases where
+    /// you need a `Text` view directly (e.g., as a return value from a computed property).
+    ///
+    /// - Parameter key: The dot-notation key for the localized string.
+    public init(localized key: String) {
+        let localizedValue = LocalizationService.shared.string(for: key)
+        self.init(localizedValue)
+    }
+}
+
+// MARK: - AppState Language Convenience
+
+extension AppState {
+    /// Gets the currently active language.
+    public var currentLanguage: LocalizationService.Language {
+        LocalizationService.shared.currentLanguage
+    }
+
+    /// Changes the active language and triggers a re-render.
+    ///
+    /// The language preference is persisted to disk and will survive app restarts.
+    /// The UI automatically re-renders with the new language strings.
+    ///
+    /// # Example
+    ///
+    /// ```swift
+    /// AppState.shared.setLanguage(.german)
+    /// ```
+    ///
+    /// - Parameter language: The language to activate.
+    public func setLanguage(_ language: LocalizationService.Language) {
+        LocalizationService.shared.setLanguage(language)
+        setNeedsRender()
+    }
+}

Sources/TUIkit/Localization/LocalizationService.swift

@@ -228,11 +228,3 @@ public final class LocalizationService: @unchecked Sendable {
     }
 }
 
-// MARK: - Convenience
-
-/// A localized string retrieved using a dot-notation key.
-///
-/// Example: `LocalizedString("button.ok")`
-public func LocalizedString(_ key: String) -> String {
-    LocalizationService.shared.string(for: key)
-}

Sources/TUIkit/Localization/LocalizedString.swift

@@ -0,0 +1,39 @@
+//  🖥️ TUIKit — Terminal UI Kit for Swift
+//  LocalizedString.swift
+//
+//  Created by LAYERED.work
+//  License: MIT
+
+// MARK: - Localized String View
+
+/// A view that displays a localized string using a dot-notation key.
+///
+/// The string is looked up in the current language from the localization service.
+/// If the key is missing, falls back to English, then to the key itself.
+///
+/// # Example
+///
+/// ```swift
+/// VStack {
+///     LocalizedString("button.ok")
+///     LocalizedString("error.invalid_input")
+/// }
+/// ```
+///
+/// Use this for all UI strings that need localization.
+public struct LocalizedString: View {
+    /// The dot-notation key for the string to display.
+    private let key: String
+
+    /// Creates a localized string view.
+    ///
+    /// - Parameter key: The dot-notation key (e.g., "button.ok", "error.invalid_input")
+    public init(_ key: String) {
+        self.key = key
+    }
+
+    public var body: some View {
+        Text(LocalizationService.shared.string(for: key))
+    }
+}
+