Must be ISO 639-1 code not ISO 3166 for langauge

In _README.md_ it says: 
> If set/detected, the subscriber’s language ISO 3166 Country Code

This is not correct. I think this should be:
> If set/detected, the subscriber’s language ISO 639-1 Language Code

See: https://mailchimp.com/help/view-and-edit-contact-languages/#Language_Codes
Ref: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
