Instead of managing the placeholder as the help text, I think we could have it just as a simple attribute of fields.Char without automatic translation. This is because I think that most of the placeholder will not have to be translated (see the example of the review which are not translatable or other common case will be format like IBAN etc.).
But also I think the placeholder could point to a field so it should be a PYSON statement, like that it can be dynamic depending on the record. For example the identifiers could have an placeholder per type.