Supported languages: https://cloud.ibm.com/docs/services/assistant?topic=assistant-language-support#language-support ???
Looks like EN > NL isn't supported (yet?):
Did you succeed to get IBM Language Translator up and running? Please report and mention your source and target language. Thanks in advance!
Am I correct in assuming that EN > NL is available via the web interface but not via the API?
The same language pairs should be available via API and the web interface. Assuming that you received the API key from IBM website, please try changing the deployment region/location to Frankfurt instead of London.