How to set Caller Language with Twilio Voice

Rasa Open Source
1

Hi, I am trying to create a German voicebot with Rasa, Twilio Voice and ngrok in the middle. The bot and the connection are working. I have set up the credentials.yml like this:

twilio:
  account_sid: "<sid>"
  auth_token:"<token>
  twilio_number: "+12345"
  language: "de-DE"

twilio_voice:
  initial_prompt: "Guten Tag. Was kann ich für Sie tun?"
  assistant_voice: "Polly.Hans"
  language: "de-DE"
  reprompt_fallback_phrase: "Das habe ich nicht verstanden."
  speech_timeout: "1"
  speech_model: "default"
  enhanced: "false"
  account_sid: "<sid>"
  auth_token: "<token>"
  twilio_number: "+12345"

The voicebot speaks German (TTS), but can not understand any intent correctly and that seems to be a problem of the Speech-to-Text expecting English instead of German. Also the “initial_prompt”-message can not be heard. In the Twilio logs you can see “en-US” as caller language. How can I change that?