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.
namespace WffmCustomization
{
[Adapter(typeof(CheckboxAdapter))]
public class Checkbox : Sitecore.Form.Web.UI.Controls.Checkbox
{
}
}
Set Assembly
and Class
fields of /sitecore/system/Modules/Web Forms for Marketers/Settings/Field Types/Simple Types/Checkbox
item to your Checkbox
class:

Then add entries with Key
set to Selected
and Not Selected
(camel case - important) in the language you need.
