Error: Preferred locale is not enabled for this site (must be en-US)

When submitting a request such as "Create Account" through the api or a client library, this error message can be returned.

This can be caused by the language settings in the email configuration not being set up to accept the language that is being sent by the request. You can find those settings at /emails
It specifically happens when the field preferred_locale is set to something the configuration does not allow.

To resolve, either add the preferred_locale you are trying to submit to the email template settings or submit the request with a value that is already accepted.

A thorough list of possible values can be found here:
https://developers.recurly.com/api/v2021-02-25/index.html#operation/create_account