feat: add multilingual localization support with in-app language switching

Package.swift

@@ -13,6 +13,7 @@ let sweetCookieKitDependency: Package.Dependency =
 
 let package = Package(
     name: "CodexBar",
+    defaultLocalization: "en",
     platforms: [
         .macOS(.v14),
     ],
@@ -98,6 +99,9 @@ let package = Package(
                 name: "CodexBarWidget",
                 dependencies: ["CodexBarCore"],
                 path: "Sources/CodexBarWidget",
+                resources: [
+                    .process("Resources"),
+                ],
                 swiftSettings: [
                     .enableUpcomingFeature("StrictConcurrency"),
                 ]),

Sources/CodexBar/About.swift

@@ -21,16 +21,16 @@ func showAbout() {
         ])
     }
 
-    let credits = NSMutableAttributedString(string: "Peter Steinberger — MIT License\n")
-    credits.append(makeLink("GitHub", urlString: "https://github.com/steipete/CodexBar"))
+    let credits = NSMutableAttributedString(string: "\(AppStrings.tr("© 2025 Peter Steinberger. MIT License."))\n")
+    credits.append(makeLink(AppStrings.tr("GitHub"), urlString: "https://github.com/steipete/CodexBar"))
     credits.append(separator)
-    credits.append(makeLink("Website", urlString: "https://codexbar.app"))
+    credits.append(makeLink(AppStrings.tr("Website"), urlString: "https://codexbar.app"))
     credits.append(separator)
-    credits.append(makeLink("Twitter", urlString: "https://twitter.com/steipete"))
+    credits.append(makeLink(AppStrings.tr("Twitter"), urlString: "https://twitter.com/steipete"))
     credits.append(separator)
-    credits.append(makeLink("Email", urlString: "mailto:peter@steipete.me"))
+    credits.append(makeLink(AppStrings.tr("Email"), urlString: "mailto:peter@steipete.me"))
     if let buildTimestamp, let formatted = formattedBuildTimestamp(buildTimestamp) {
-        var builtLine = "Built \(formatted)"
+        var builtLine = AppStrings.fmt("Built %@", formatted)
         if let gitCommit, !gitCommit.isEmpty, gitCommit != "unknown" {
             builtLine += " (\(gitCommit)"
             #if DEBUG
@@ -68,7 +68,7 @@ private func formattedBuildTimestamp(_ timestamp: String) -> String? {
     let formatter = DateFormatter()
     formatter.dateStyle = .medium
     formatter.timeStyle = .short
-    formatter.locale = .current
+    formatter.locale = AppStrings.locale
     return formatter.string(from: date)
 }
 

Sources/CodexBar/AppLanguage.swift

@@ -0,0 +1,57 @@
+import Foundation
+
+enum AppLanguage: String, CaseIterable, Identifiable {
+    case system
+    case english = "en"
+    case simplifiedChinese = "zh-Hans"
+    case traditionalChinese = "zh-Hant"
+
+    static let userDefaultsKey = "appLanguage"
+
+    var id: String {
+        self.rawValue
+    }
+
+    var localizationIdentifier: String? {
+        switch self {
+        case .system:
+            nil
+        case .english:
+            "en"
+        case .simplifiedChinese:
+            "zh-Hans"
+        case .traditionalChinese:
+            "zh-Hant"
+        }
+    }
+
+    var locale: Locale {
+        switch self {
+        case .system:
+            .autoupdatingCurrent
+        case .english:
+            Locale(identifier: "en")
+        case .simplifiedChinese:
+            Locale(identifier: "zh-Hans")
+        case .traditionalChinese:
+            Locale(identifier: "zh-Hant")
+        }
+    }
+
+    var displayName: String {
+        switch self {
+        case .system:
+            "System"
+        case .english:
+            "English"
+        case .simplifiedChinese:
+            "简体中文"
+        case .traditionalChinese:
+            "繁體中文"
+        }
+    }
+
+    static func resolve(from defaults: UserDefaults) -> AppLanguage {
+        AppLanguage(rawValue: defaults.string(forKey: self.userDefaultsKey) ?? "") ?? .system
+    }
+}

Sources/CodexBar/AppStringResources.swift

@@ -0,0 +1,37 @@
+import Foundation
+
+enum AppStringResources {
+    private static let supportedLocalizations = ["en", "zh-Hans", "zh-Hant"]
+
+    static func localizedString(for key: String, table: String, language: AppLanguage) -> String {
+        if let localized = self.localizedString(
+            for: key,
+            table: table,
+            localization: language.localizationIdentifier)
+        {
+            return localized
+        }
+        if let english = self.localizedString(for: key, table: table, localization: "en") {
+            return english
+        }
+        return key
+    }
+
+    private static func localizedString(for key: String, table: String, localization: String?) -> String? {
+        guard let bundle = self.bundle(for: localization) else { return nil }
+        let sentinel = "__codexbar_missing_translation__"
+        let value = bundle.localizedString(forKey: key, value: sentinel, table: table)
+        return value == sentinel ? nil : value
+    }
+
+    private static func bundle(for localization: String?) -> Bundle? {
+        guard let localization else { return Bundle.module }
+        guard self.supportedLocalizations.contains(localization),
+              let bundleURL = Bundle.module.resourceURL?.appendingPathComponent("\(localization).lproj"),
+              FileManager.default.fileExists(atPath: bundleURL.path)
+        else {
+            return nil
+        }
+        return Bundle(url: bundleURL)
+    }
+}