422: Unprocessable Entity, locale is invalid.

Problem/Motivation

I run the latest released version from .nl domain with Drupal configured into Dutch language. When trying to do a payment, I get an error message. The drupal log message reports:
Error executing API call (422: Unprocessable Entity): The locale is invalid. Field: locale.

I think the locale for my environment needs to be set to 'nl-NL', but don't know how I can do this.

Proposed resolution

set a locale overwrite in the configuration.
As a temporary solution, is there a way to set the locale from code?