使用替代语言代码的异步语音识别响应没有语言代码

我正在尝试使用新的测试版替代语言功能 https://cloud.google.com/speech-to-text/docs/multiple-languages允许在创建转录作业时提供一组语言，并返回检测到的语言以及该语言的转录结果。

当我从文档页面（同步）运行代码示例时，一切运行正常，并且检测到的语言代码在结果中返回：

from google.cloud import speech_v1p1beta1 as speech
client = speech.SpeechClient()

speech_file = 'resources/multi.wav'
first_lang = 'en-US'
second_lang = 'es'

with open(speech_file, 'rb') as audio_file:
    content = audio_file.read()

audio = speech.types.RecognitionAudio(content=content)

config = speech.types.RecognitionConfig(
    encoding=speech.enums.RecognitionConfig.AudioEncoding.LINEAR16,
    sample_rate_hertz=44100,
    audio_channel_count=2,
    language_code=first_lang,
    alternative_language_codes=[second_lang])

print('Waiting for operation to complete...')
response = client.recognize(config, audio)

for i, result in enumerate(response.results):
    alternative = result.alternatives[0]
    print(result.language_code)  # this prints 'en-US'
    print('-' * 20)
    print('First alternative of result {}: {}'.format(i, alternative))
    print(u'Transcript: {}'.format(alternative.transcript))

但是当我尝试异步模式时，语言代码不会随结果一起返回：

from google.cloud import speech_v1p1beta1 as speech
client = speech.SpeechClient()

gs_url = 'gs://my-bucket-name/multi.wav'
first_lang = 'en-US'
second_lang = 'es'

audio = speech.types.RecognitionAudio(uri=gs_url)

config = speech.types.RecognitionConfig(
    encoding=speech.enums.RecognitionConfig.AudioEncoding.LINEAR16,
    sample_rate_hertz=44100,
    audio_channel_count=2,
    language_code=first_lang,
    alternative_language_codes=[second_lang])

print('Waiting for operation to complete...')
operation = client.long_running_recognize(config, audio)
response = operation.result(timeout=40)

for i, result in enumerate(response.results):
    alternative = result.alternatives[0]
    print(result.language_code)  # this prints nothing! result.language_code is empty string 
    print('-' * 20)
    print('First alternative of result {}: {}'.format(i, alternative))
    print(u'Transcript: {}'.format(alternative.transcript))

尽管出现这种行为文档明确说明 https://cloud.google.com/speech-to-text/docs/multiple-languages#enabling_language_recognition_in_audio_transcription_requests:

Speech-to-Text 支持所有语音识别方法的替代语言代码：speech:recognize、speech:longrunningrecognize 和 Streaming。

关于如何为异步转录请求获取检测到的语言代码有什么想法吗？

这是我的谷歌图书馆版本 https://i.stack.imgur.com/M3Fs4.png