GET request with the "Accept-Language" header being blocked

I have the following code:

1async function fetchNamespace(locale: string, namespace: string): Promise<Object> {
2  const url = `https://myapiurl.com/v1/translation/${namespace}?format=flat`
3  const headers = { "Accept-Language": locale } 
4  const response = await fetch(url, { method: 'GET', headers: headers })
5
6  const content = await response.json() as any
7  return content[Object.keys(content)[0]]
8}

It gives me the following error:

Access to fetch at 'https://myapiurl.com/v1/translation/nes?format=flat' from origin 'null' has been blocked by CORS policy: Request header field accept-language is not allowed by Access-Control-Allow-Headers in preflight response

Is there a way to allow the "Accept-Language" header somewhere? I know the header is the problem because:

• I've tried sending the same request without the headers and it works
• I've sent the same request over CURL with Origin set to null, and it also works.

The CURL command that's working:

1curl -X GET "https://myapi.com/v1/translation/nes?format=flat" \
2-H "Accept-Language: en_US" \
3-H "Origin: null"

Any ideas?