How to translate active record model validations

The translation for the title would be:

nl:
  activerecord:
    errors:
      template:
        header:
          one:   "1 error prohibited this %{model} from being saved"
          other: "%{count} errors prohibited this %{model} from being saved"
        body:    "There were problems with the following fields:"

For translating the error messages, Rails will use the following order of translations:

activerecord.errors.models.user.attributes.name.blank
activerecord.errors.models.user.blank
activerecord.errors.messages.blank
errors.attributes.name.blank
errors.messages.blank

So you could add:

nl:
  activerecord:
    errors:
      models:
        user:
          attributes:
            email:
              blank: "foo blank in nl bar baz"

It’s documented in the Rails Internationalization (I18n) API Guide, which might give you some more insight.