In this section of /secure/register.aspx.cs there's a hard-coded message, that should be using a resource

   if (siteSettings.RequireEnterEmailTwiceOnRegistration)
            {
                Panel divConfirmEmail = (Panel)CreateUserWizardStep1.ContentTemplateContainer.FindControl("divConfirmEmail");
                CompareValidator EmailCompare = (CompareValidator)CreateUserWizardStep1.ContentTemplateContainer.FindControl("EmailCompare");
                RequiredFieldValidator ConfirmEmailRequired = (RequiredFieldValidator)CreateUserWizardStep1.ContentTemplateContainer.FindControl("ConfirmEmailRequired");

                //ConfirmEmailRequired
                if ((divConfirmEmail != null) && (EmailCompare != null))
                {
                    divConfirmEmail.Visible = true;
                    EmailCompare.ErrorMessage = Resource.RegisterCompareEmailWarning;
                    EmailCompare.Enabled = true;
                    EmailCompare.EnableClientScript = true;

                    ConfirmEmailRequired.ErrorMessage = "Confirm Email is required.";
                    ConfirmEmailRequired.Enabled = true;

                }
 

We've change this to = Resource.RegisterCompareEmailRequired;