Hi,
The language field in Acymailing is a core field and can not be disabled or changed.
I'm not familiar with Balbooa forms. The documentation (https://www.balbooa.com/joomla-forms-documentation/acymailing-integration) explains you have to match the fields between your form and Acymailing. Maybe you can find something there that you can disable.
With other forms (like RSForm) there are no issues with this field. So you either have something in your form that needs to be changed or you need to contact the Balbooa devs to ask them what is going on.