Hey @MarcelB,
Thank you for following up and providing more detail. Looking at the POST body that you’re using more closely, it looks like the language_interpretation object is not within a settings object.
We can see that this is required in the Create a Meeting API documentation as the language_interpretation property is under the settings object.
Here’s an example of using the settings object

Here’s an example of what that would look like with your request:
{
"template_id": "s2aXj0lYStSnKNUAUfTDww",
"topic": "Immersion : session #77 - Bodhi",
"start_time": "2021-04-13T20:00:00",
"timezone": "Europe\/Paris",
"duration": 90,
"password": "XXXX",
"settings": {
"language_interpretation": {
"enable": "true",
"interpreters": [
{
"email": "marcel-web@brouillet.org",
"languages": "US,FR"
}
]
}
}
}
Please try placing the language_interpretation object within a settings object.
Let me know if that helps! 
Thanks,
Max