The official Ginger Payments and Kassa Compleet extensions for Magento 1.x pass the same customer data to «POST /v1/orders/» besides the «customer/locale» format

The Ginger Payments extension passes a customer’s locale as nl.
The Kassa Compleet extension passes a customer’s locale as nl_NL.

Both formats are correct by the Kassa Compleet’s JSON Schema of a «POST /v1/orders/» request.