Pass language defined for individual form to MailChimp

I have two forms. One is used on a German part of my website and the other one is used on the English part. I want to pass on the language to MailChimp so it is saved with the subscriber in the default language field. I already tried to accomplish this with custom fields and using the language code values (de & en) and passing it to MC_LANGUAGE but it did not work. How can I achieve this?