openAI API curl statement: How to feed the messages json array with variables?

Question

I am struggling with the gpt-4 API and the new messages json array which is used to feed the prompt. How can I combine fixed text with variables in the "content" section ? For Example "Please translate the following text:" $selectedText "into" $language. The variables are defined - even the fixed text has been put into variables.

curl https://api.openai.com/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $api_key" \
  -d '{
  "model": "gpt-4",
  "messages": [{"role": "user", "content": "'$intro $selectedText $into $language'"}],
  "temperature": 0.7
  }'

Error message:

curl https://api.openai.com/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $api_key" \
  -d '{"model": "gpt-4", "messages": [{"role": "user", "content": "'$intro $selectedText $into $language'"}], "temperature": 0.7}'
{
    "error": {
        "message": "We could not parse the JSON body of your request. (HINT: This likely means you aren't using your HTTP library correctly. The OpenAI API expects a JSON payload, but what was sent was not valid JSON. If you have trouble figuring out how to fix this, please contact us through our help center at help.openai.com.)",
        "type": "invalid_request_error",
        "param": null,
        "code": null
    }
}
curl: (3) URL rejected: Malformed input to a URL function
curl: (6) Could not resolve host: into
curl: (3) unmatched close brace/bracket in URL position 8:
german"}], "temperature": 0.7}