"TemplateLanguage" is not set while "TemplateErrorReporting" or "TemplateErrorDeliver"

Problem/Motivation

I freshly installed the module and also setup the template error setting and went straight to the send test email and get the following error:

"TemplateLanguage" is not set while "TemplateErrorReporting" or "TemplateErrorDeliver" properties are

More verbose:

Array
(
    [Messages] => Array
        (
            [0] => Array
                (
                    [Status] => error
                    [Errors] => Array
                        (
                            [0] => Array
                                (
                                    [ErrorIdentifier] => 36fa5b18-66d9-4b6d-XXXX-438a9a27c4c6
                                    [ErrorCode] => send-0016
                                    [StatusCode] => 400
                                    [ErrorMessage] => "TemplateLanguage" is not set while "TemplateErrorReporting" or "TemplateErrorDeliver" properties are.
                                    [ErrorRelatedTo] => Array
                                        (
                                            [0] => TemplateLanguage
                                            [1] => TemplateErrorDeliver
                                            [2] => TemplateErrorReporting
                                        )

                                )

                        )

                )

        )
)

I have no idea why that happens, I can only enable the TemplateErrorReporting, but not the Language it will happen in. Thus I guess there should be some kind of default language 'en' ?? when this is enabled. The Drupal I use is multilanguage, but not that well setup. Maybe there is a problem, when the mailjet API tries to fetch the default language from the drupal system. Either way, I guess a default should be provided, when the Drupal itself is not that helpful.