Removing course language setting ( course[locale] ) via API

[ThayerMerritt]

Our team is looking to standardize all courses to have the language setting follow a user's default language selection, as opposed to enforce at the course setting level. 

While most are set as "default" or "not set," we're finding that some of our blueprint courses already have the language set "English."

I'm having trouble removing the value using API. I've been successful in applying a setting (en, fr, de -- as examples), but having a blank value (or "null" "default" "0") all return the following json response

{
    "errors": {
        "locale": [
            {
                "attribute": "locale",
                "message": "is not included in the list",
                "type": "inclusion"
            }
        ]
    }
}

I'm unable to find the list of valid values or how to remove the value once set in the documentation.

Any assistance on this would be appreciated!

Thanks!