언어 설정 화면 추가 및 언어 헤더 적용

설정에서 시스템/한국어/영어/일본어 선택을 지원한다.

선택 시 Accept-Language 헤더와 UI locale을 즉시 반영한다.

언어 변경 후 스플래시를 거쳐 메인으로 소프트 재시작한다.

SodaLive/Sources/Settings/Language/Services/LanguageService.swift

@@ -0,0 +1,108 @@
+//
+//  LanguageService.swift
+//  SodaLive
+//
+//  Created by Junie (AI) on 2025/12/16.
+//
+
+import Foundation
+
+enum LanguageHeaderProvider {
+    private static let defaultsKey = "app.language"
+
+    // 현재 헤더 코드(ko|en|ja). 앱 시작 시 initialize()로 동기 설정.
+    static var current: String = {
+        // 초기값은 OS 선호 기반으로 계산
+        return mapPreferredToSupported(Locale.preferredLanguages.first)
+    }()
+
+    /// 앱 시작 직후 동기 초기화: 저장된 설정이 있으면 그 값을, 없으면 OS 선호를 사용.
+    static func initialize() {
+        if let raw = UserDefaults.standard.string(forKey: defaultsKey),
+           let option = LanguageOption(rawValue: raw),
+           option != .system {
+            // 명시 선택 값
+            current = option.rawValue
+            return
+        }
+        // 시스템 기본 또는 저장값 없음 → OS 선호 매핑
+        current = mapPreferredToSupported(Locale.preferredLanguages.first)
+    }
+
+    private static func mapPreferredToSupported(_ tag: String?) -> String {
+        guard let tag = tag else { return "en" }
+        if tag.hasPrefix("ko") { return "ko" }
+        if tag.hasPrefix("ja") { return "ja" }
+        return "en"
+    }
+}
+
+actor LanguageService {
+    private let repository: LanguageRepository
+    private let environment: LanguageEnvironment
+
+    init(repository: LanguageRepository, environment: LanguageEnvironment) {
+        self.repository = repository
+        self.environment = environment
+    }
+
+    func bootstrap() async {
+        let saved = (try? await repository.load()) ?? .system
+        await applyLanguage(saved)
+    }
+
+    func applyLanguage(_ option: LanguageOption) async {
+        do { try await repository.save(option) } catch { DEBUG_LOG("[Language] save failed: \(error)") }
+        let locale = resolveLocale(option)
+        let header = resolveHeader(option, locale: locale)
+        LanguageHeaderProvider.current = header
+        await MainActor.run { [weak environment] in
+            environment?.locale = locale
+        }
+        DEBUG_LOG("[Language] changed to option=\(option.rawValue) locale=\(locale.identifier) header=\(header)")
+    }
+
+    func currentLocale() async -> Locale {
+        await MainActor.run { environment.locale }
+    }
+
+    func currentLanguageCodeForHeader() async -> String { LanguageHeaderProvider.current }
+
+    // MARK: - Private
+    private func resolveLocale(_ option: LanguageOption) -> Locale {
+        switch option {
+        case .system:
+            guard let first = Locale.preferredLanguages.first else { return Locale(identifier: "en_US") }
+            return mapToSupported(first)
+        case .ko: return Locale(identifier: "ko_KR")
+        case .en: return Locale(identifier: "en_US")
+        case .ja: return Locale(identifier: "ja_JP")
+        }
+    }
+
+    private func resolveHeader(_ option: LanguageOption, locale: Locale) -> String {
+        switch option {
+        case .system:
+            let id = locale.identifier
+            if id.hasPrefix("ko") { return "ko" }
+            if id.hasPrefix("ja") { return "ja" }
+            return "en"
+        case .ko: return "ko"
+        case .en: return "en"
+        case .ja: return "ja"
+        }
+    }
+
+    private func mapToSupported(_ tag: String) -> Locale {
+        if tag.hasPrefix("ko") { return .init(identifier: "ko_KR") }
+        if tag.hasPrefix("ja") { return .init(identifier: "ja_JP") }
+        return .init(identifier: "en_US")
+    }
+}
+
+// Composition root for language feature
+enum LanguageContainer {
+    static let environment = LanguageEnvironment()
+    static let repository: LanguageRepository = UserDefaultsLanguageRepository()
+    static let service = LanguageService(repository: repository, environment: environment)
+}