feat(settings): add application language selection

ShadowsocksX-NG2/App/Composition/AppLanguagePreferences.swift

@@ -0,0 +1,30 @@
+import Combine
+import Foundation
+
+/// 应用语言覆盖使用 macOS 的应用级 AppleLanguages 偏好；不写全局偏好。
+/// Bundle 在进程内缓存语言选择，因此保存后由下次启动统一应用。
+@MainActor
+final class AppLanguagePreferences: ObservableObject {
+  @Published private(set) var selection: AppLanguage
+
+  private let defaults: UserDefaults
+
+  init(
+    defaults: UserDefaults = .standard,
+    applicationID: String = "com.qiuyuzhou.ShadowsocksX-NG2"
+  ) {
+    self.defaults = defaults
+    let languages =
+      defaults.persistentDomain(forName: applicationID)?["AppleLanguages"] as? [String]
+    selection = languages?.first.flatMap(AppLanguage.init(rawValue:)) ?? .automatic
+  }
+
+  func select(_ language: AppLanguage) {
+    if language == .automatic {
+      defaults.removeObject(forKey: "AppleLanguages")
+    } else {
+      defaults.set([language.rawValue], forKey: "AppleLanguages")
+    }
+    selection = language
+  }
+}

ShadowsocksX-NG2/App/Presentation/AppLanguage.swift

@@ -0,0 +1,31 @@
+import Foundation
+
+enum AppLanguage: String, CaseIterable {
+  case automatic
+  case simplifiedChinese = "zh-Hans"
+  case english = "en"
+
+  func currentValueTitle(in bundle: Bundle = .main) -> String {
+    self == .automatic ? localizedName(in: bundle) : nativeName
+  }
+
+  func menuTitle(in bundle: Bundle = .main) -> String {
+    "\(nativeName) · \(localizedName(in: bundle))"
+  }
+
+  private var nativeName: String {
+    switch self {
+    case .automatic: "Auto Detect"
+    case .simplifiedChinese: "简体中文"
+    case .english: "English"
+    }
+  }
+
+  private func localizedName(in bundle: Bundle) -> String {
+    switch self {
+    case .automatic: String(localized: "自动检测", bundle: bundle)
+    case .simplifiedChinese: String(localized: "简体中文", bundle: bundle)
+    case .english: String(localized: "英文", bundle: bundle)
+    }
+  }
+}

ShadowsocksX-NG2/App/Resources/Localizable.xcstrings

@@ -1906,6 +1906,61 @@
           }
         }
       }
+    },
+    "语言": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Language"
+          }
+        }
+      }
+    },
+    "下次启动生效": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Takes effect next launch"
+          }
+        }
+      }
+    },
+    "自动检测": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Auto Detect"
+          }
+        }
+      }
+    },
+    "简体中文": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Simplified Chinese"
+          }
+        }
+      }
+    },
+    "英文": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "English"
+          }
+        }
+      }
     }
   },
   "version": "1.0"

ShadowsocksX-NG2/App/UI/Settings/LanguageSettingsRow.swift

@@ -0,0 +1,36 @@
+import SwiftUI
+
+struct LanguageSettingsRow: View {
+  @StateObject private var preferences = AppLanguagePreferences()
+
+  var body: some View {
+    HStack(spacing: 16) {
+      VStack(alignment: .leading, spacing: 2) {
+        Text("语言")
+        Text("下次启动生效")
+          .font(.caption)
+          .foregroundStyle(.secondary)
+      }
+      Spacer(minLength: 16)
+      Picker(
+        "语言",
+        selection: Binding(
+          get: { preferences.selection },
+          set: { preferences.select($0) }),
+        content: {
+          ForEach(AppLanguage.allCases, id: \.self) { language in
+            Text(verbatim: language.menuTitle()).tag(language)
+          }
+        },
+        currentValueLabel: {
+          Text(verbatim: preferences.selection.currentValueTitle())
+        }
+      )
+      .pickerStyle(.menu)
+      .labelsHidden()
+      .fixedSize()
+      .accessibilityIdentifier("appLanguagePicker")
+    }
+    .padding(.vertical, 4)
+  }
+}

ShadowsocksX-NG2/App/UI/Settings/SettingsView.swift

@@ -68,6 +68,7 @@ struct SettingsView: View {
         .toggleStyle(.switch)
       }
       .padding(.vertical, 4)
+      LanguageSettingsRow()
     } header: {
       sectionHeader("常规")
     }

ShadowsocksX-NG2/Tests/Settings/AppLanguagePreferencesTests.swift

@@ -0,0 +1,57 @@
+import Foundation
+import Testing
+
+@testable import ShadowsocksX_NG2
+
+@MainActor
+struct AppLanguagePreferencesTests {
+  @Test func missingApplicationOverrideUsesAutomatic() throws {
+    let suite = "language-tests-\(UUID().uuidString)"
+    let defaults = try #require(UserDefaults(suiteName: suite))
+    defer { defaults.removePersistentDomain(forName: suite) }
+
+    let preferences = AppLanguagePreferences(defaults: defaults, applicationID: suite)
+
+    #expect(preferences.selection == .automatic)
+  }
+
+  @Test func languageSelectionSurvivesRelaunchAndAutomaticRemovesOverride() throws {
+    let suite = "language-tests-\(UUID().uuidString)"
+    let defaults = try #require(UserDefaults(suiteName: suite))
+    defer { defaults.removePersistentDomain(forName: suite) }
+    defaults.set(true, forKey: "silentLaunch")
+    let preferences = AppLanguagePreferences(defaults: defaults, applicationID: suite)
+
+    for language in [AppLanguage.english, .simplifiedChinese] {
+      preferences.select(language)
+      #expect(preferences.selection == language)
+      #expect(
+        AppLanguagePreferences(defaults: defaults, applicationID: suite).selection == language)
+      #expect(defaults.stringArray(forKey: "AppleLanguages") == [language.rawValue])
+    }
+
+    preferences.select(.automatic)
+    #expect(
+      AppLanguagePreferences(defaults: defaults, applicationID: suite).selection == .automatic)
+    #expect(defaults.persistentDomain(forName: suite)?["AppleLanguages"] == nil)
+    #expect(defaults.bool(forKey: "silentLaunch"))
+  }
+
+  @Test func menuNamesUseNativePrefixAndCurrentInterfaceTranslation() throws {
+    let appBundle = AppArtifact.bundle
+    let english = try #require(
+      Bundle(url: appBundle.bundleURL.appendingPathComponent("Contents/Resources/en.lproj")))
+    let chinese = try #require(
+      Bundle(url: appBundle.bundleURL.appendingPathComponent("Contents/Resources/zh-Hans.lproj")))
+
+    #expect(AppLanguage.automatic.menuTitle(in: english) == "Auto Detect · Auto Detect")
+    #expect(AppLanguage.simplifiedChinese.menuTitle(in: english) == "简体中文 · Simplified Chinese")
+    #expect(AppLanguage.english.menuTitle(in: english) == "English · English")
+    #expect(AppLanguage.automatic.menuTitle(in: chinese) == "Auto Detect · 自动检测")
+    #expect(AppLanguage.simplifiedChinese.menuTitle(in: chinese) == "简体中文 · 简体中文")
+    #expect(AppLanguage.english.menuTitle(in: chinese) == "English · 英文")
+    #expect(AppLanguage.automatic.currentValueTitle(in: english) == "Auto Detect")
+    #expect(AppLanguage.simplifiedChinese.currentValueTitle(in: english) == "简体中文")
+    #expect(AppLanguage.english.currentValueTitle(in: chinese) == "English")
+  }
+}

docs/design/app-language-selection.md

@@ -0,0 +1,40 @@
+# 应用语言选择
+
+## 范围
+
+在设置的「常规」末尾新增语言下拉框，提供自动检测、简体中文、英文。
+默认自动检测。保存后下次启动生效，不自动退出或重启应用。
+本文件记录本次需求与实现取舍，不作为项目当前状态的持续维护文档。
+
+## 展示
+
+- 收起时显示所选项的单个名称：自动检测使用当前界面语言，语言项使用自称。
+- 展开时显示「自称 · 当前界面语言名称」。重复名称保留。
+- 自动检测的固定前缀为 `Auto Detect`。
+- 顺序为自动检测、简体中文、英文。
+- 行标题为「语言」/ `Language`，说明为「下次启动生效」/ `Takes effect next launch`。
+
+| 选项 | 中文界面 | 英文界面 |
+| --- | --- | --- |
+| 自动检测 | Auto Detect · 自动检测 | Auto Detect · Auto Detect |
+| 简体中文 | 简体中文 · 简体中文 | 简体中文 · Simplified Chinese |
+| 英文 | English · 英文 | English · English |
+
+## 偏好与生效
+
+语言选择只影响本应用。显式选择写入应用级 `AppleLanguages`，自动检测移除该覆盖。
+自动检测沿用 macOS 对应用支持语言的匹配；开发语言为简体中文。
+不改全局语言、地区格式、代理设置或代理运行状态。
+应用现有 SwiftUI 与 Foundation 本地化在下次启动读取同一应用偏好。
+本次不迁移既有未本地化文案。
+
+## Module 与测试 seam
+
+`AppLanguagePreferences` 的 Interface 提供当前选择与保存选择。
+该 Module 隐藏应用偏好域的读取及语言覆盖的写入。
+`AppLanguage` 提供收起名称与双语菜单名称。
+使用系统原生 Picker，独立提供当前值标签。
+
+测试通过上述 Interface 检查缺失偏好、持久化往返、清除覆盖与双语名称。
+测试使用进程私有 UserDefaults 域。
+启动解析另外通过隔离的小型应用 bundle 验证，覆盖自动检测与显式覆盖。