@sam I think for a locally run language detection container the detection only works with websockets i.e the SDK.
This is mentioned in the FAQ section of the speech containers. Although language detection status and ready API's seem to support REST API calls.
You can test your local container by running antsu language detection client and check if your local endpoint is working. This command should work in this case.
docker run --rm -ti antsu/on-prem-client:latest ./speech-to-text-with-languagedetection-client ./audio/LanguageDetection_en-us.wav --host localhost --lport 5003 --windows
The error with speech to text recognition is because i am not running speech to text container and not passing sport=5000
in the above command.