工具类：

let UserLanguage = "UserLanguage"
let AppleLanguages = "AppleLanguages"

class LanguageHelper: NSObject {
    static let share = LanguageHelper()

    let def = UserDefaults.standard

    var bundle : Bundle?

    class func getString(_ key:String) -> String{
        let bundle = LanguageHelper.share.bundle
        let str = bundle?.localizedString(forKey: key, value: nil, table: nil)
    
        return str!
    }

    func initUserLanguage() {
        var string: String = def.value(forKey: UserLanguage) as? String ?? ""
        if string == "" {
            if let languages = def.object(forKey: AppleLanguages) as? NSArray {
                if languages.count > 0 {
                    if let current = languages.object(at: 0) as? String {
                        string = current
                        def.set(current.components(separatedBy: "-").first, forKey: UserLanguage)
                        def.synchronize()
                    }
                }
            }
        }
    
        switch string {
        case LangKey.ES_latam:
            break
        default:
            if let str = string.components(separatedBy: "-").first {
                string = str
            }
            break
        }
        var path = Bundle.main.path(forResource:string , ofType: "lproj")
        if path == nil {
            path = Bundle.main.path(forResource:"en" , ofType: "lproj")
        }
        bundle = Bundle(path: path!)
    }

    func setLanguage(_ langeuage:String) {
        let path = Bundle.main.path(forResource:langeuage , ofType: "lproj")
        bundle = Bundle(path: path!)
        def.set(langeuage, forKey: UserLanguage)
        def.synchronize()
    }
}

使用方法：

1. 首先需要实例化工具类里的bundle，使用initUserLanguage方法，可以把该方法写在AppDelegate中。

2. 其次写一个扩展或者类方法，调用getString方法国际化字符串（即：使用该方法代替原有的国际化方法），当然也可以每次直接使用getString方法，例如：

class func string(_ key: String) -> String {
    return LanguageHelper.getString(key)
}

3. 接下来就是最重要的部分，做一个语言选择的界面或控件，利用工具类的方法实现这个类的功能，并通过监听实现以创建的视图的语言国际化，没有创建的视图在创建的时候会通过第2步的方法实现语言国际化。代码如下：（以界面为例，这是用故事版创建的一个tableview的界面）

class LanguageViewController: UITableViewController {

@IBOutlet var doneBtn: UIBarButtonItem!

override func viewDidLoad() {
    super.viewDidLoad()
    self.initUI()
    self.initLocalizableString()
}

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

private var currentLang: String!
private var selectLang: String!
private var dataArray: Array<String> = Array()

private func initUI() {
    //init
    self.dataArray = LanguageModel.getLangArr()
    //当前语言获取
    self.currentLang = UserDefaults.standard.value(forKey: UserLanguage) as? String
    
    if Utils.isEmpty(currentLang) {//最开始没有设置过语言时，获取系统语言
        if let langs = UserDefaults.standard.object(forKey: AppleLanguages) as? Array<String> {
            if langs.count > 0 {
                self.currentLang = langs[0]
            }
        }
    }
    //特殊处理
    switch self.currentLang {
    case LangKey.ES_latam:
        break
    default:
        self.currentLang = self.currentLang.components(separatedBy: "-").first
        break
    }
    //UI显示数据
    self.currentLang = LanguageModel.getCurrentLang(currentLang)
    self.selectLang = self.currentLang
    
    self.doneBtn.action = #selector(doneClick)
    self.doneBtn.target = self
    self.doneBtn.titleFont(UIFont.systemFont(ofSize: 13))
}

@objc func doneClick(_ button: UIBarButtonItem) {
    self.setCurrLanguage()
}

private func initLocalizableString() {
    self.navigationItem.title = i18n(I18N.Identifier(rawValue: "id_navigation_title_language")!)
    self.doneBtn.title = i18n(I18N.Identifier(rawValue: "id_navigation_right_button_done")!)
}
private func setCurrLanguage() {
    let code = LanguageModel.getCurrentLangCode(self.selectLang)
    //把selectLang转换为当前语言所对应的字符串id
//        let langStr = LanguageModel.getCurrentLangString(self.selectLang)
    //设置语言
    LanguageHelper.share.setLanguage(code)
    //把selectLang转换为当前语言所对应的字符串,赋值给currentLang
    self.currentLang = self.selectLang
    //国际化selectLang
//        self.selectLang = langStr
    //重新走一遍I18N方法
    self.initLocalizableString()
    self.tableView.reloadData()
    //发出语言监听
    NotificationCenter.default.post(name: NSNotification.Name(rawValue: ChangeLanguage), object: nil)
}

// MARK: - Table view data source

override func numberOfSections(in tableView: UITableView) -> Int {
    // #warning Incomplete implementation, return the number of sections
    return 1
}

override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    // #warning Incomplete implementation, return the number of rows
    
    return dataArray.count
}

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let id = "LanguageCell"
    
    let cell = tableView.dequeueReusableCell(withIdentifier: id, for: indexPath)
    let str = dataArray[indexPath.row]
    
    cell.textLabel?.text = str
    
    if str == self.currentLang {
        cell.accessoryType = .checkmark
    }else {
        cell.accessoryType = .none
    }
    
    return cell
}

override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    self.setAllCell()
    let cell = tableView.cellForRow(at: indexPath)
    cell?.accessoryType = .checkmark
    self.selectLang = cell?.textLabel?.text
}

private func setAllCell() {
    for i in 0..<langKey.count {
        let cell = tableView.cellForRow(at: IndexPath(row: i, section: 0))
        cell?.accessoryType = .none
    }
}

}

这个工具对于不同的app需要作一些改变，请自行斟酌！