Please help: Conversation Voice Assist other language than English gives error

Trying to get NLD language to work for 3 days, but it keeps failing.

Created testing.yaml in “**\config\custom_sentences\nl**”

language: "nl"
intents:
  EenTest:
    data:
      - sentences:
        - "Hoe gaat het met je"

In configuration.yaml:

conversation:

intent_script:
  EenTest:
    speech:
      text: "De test is geslaagd"

Set in my profile in Home Assistant Language to Netherlands
(ps. even tryed setting phone language and Hass to NL or English, but to no result).

The Assist does respond in NL language (asking to talk in NL and errors).
But all responses are “Oeps er is een fout opgetreden” (English: Oops something went wrong).

ps. Assist in English does work, but I really would like NL to work.

Q: Is there an Debug log option for Conversation Voice Assist?

Please help.