You can override
Checkbox class and add
Adapter attribute. It's there in WFFM already. I have no idea why it's not used by default.
public class Checkbox : Sitecore.Form.Web.UI.Controls.Checkbox
Class fields of
/sitecore/system/Modules/Web Forms for Marketers/Settings/Field Types/Simple Types/Checkbox item to your
Then add entries with
Key set to
Not Selected (camel case - important) in the language you need.