使用iOS TTS无法更改AVSpeechSynthesisVoice语言环境

Question

使用 Xcode 7.2 ，不同的部署目标（例如7.1和9.2）和不同的虚拟设备（例如iPhone 6s plus和iPhone 5s）我无法切换语音TTS语言。这是我的代码：

  z=theano.function([x],w_v_bias)
  z(datax[0])

两种文本都被阅读，但仅限于设备的标准语言。

此外，当我运行代码时出现以下错误：

import UIKit
import AVFoundation

class ViewController: UIViewController {
    let synth = AVSpeechSynthesizer()
    var myUtterance = AVSpeechUtterance()

    func speak(locale: String, text: String) {
        if let voice = AVSpeechSynthesisVoice(language: locale) {
            myUtterance.voice = voice

            // I read that the delays might cure the issue,
            // but they don't. Language switching does not
            // work with or without this.
            myUtterance.preUtteranceDelay = 0.005
            myUtterance.postUtteranceDelay = 0.005

            // I read that an empty text cures the issue,
            // but is doesn't. Language switching does not
            // work with or without this.
            for t in [" ", text] {
                myUtterance = AVSpeechUtterance(string: t)
                synth.speakUtterance(myUtterance)
            }
        } else {
            debugPrint("bad voice")
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        speak("de-DE", text: "Ich blicke auf einen blauen Himmel!")
        speak("en-US", text: "This is a text, which I like very much!")
    }
}

在iPhone 4上运行应用程序（不是虚拟的，它是iOS 7.1.2的真实设备）应用程序启动并且不会发生此错误，但TTS根本不起作用。

提前多多感谢