To customize the copy within the Form you will need to set a global variable called _mpi_publicform_customizations,  this needs to be an Object. See the example below. 

This variable doesn't need all the fields to be present. If a field isn't present then the default value is used. The requiredLabel on the fields is only used when the field is required. For instance, if you make the 'upload' box required then it will show when this is set. Here's a example of some custom copy for the form:


var _mpi_publicform_customizations = {
  "titleText": "Votre histoire",
  "submitButtonText": "Soumettre votre histoire",
  "successMessage": "Success",
  "successMessageCloseButton": "Okay",
  "termsText": "Termes ou service",
  "fields": {
    "author": {
      "label": "Nom d'utilisateur",
      "value": "",
      "placeholder": "Ce nom sera visible publiquement",
      "errorEmpty": "The field 'Display Name' is empty!",
      "errorLength": "The field 'Display Name' must to be longer! (5 or more
letters)",
      "requiredLabel": ""
    },
    "email": {
      "label": "Adresse e-mail",
      "value": "",
      "placeholder": "Votre adresse e-mail ne sera pas visible
publiquement",
      "errorEmpty": "The field 'Email Address' is empty!",
      "errorInvalid": "The field 'Email Address' is invalid!",
      "requiredLabel": ""
    },
    "caption": {
      "label": "Légende",
      "value": "",
      "placeholder": "Ton histoire!",
      "errorLength": "The field 'Story' is empty!",
      "requiredLabel": ""
    },
    "upload": {
      "label": "Ajouter une image",
      "errorEmpty": "You need to add image!",
      "buttonText": "Chercher un document",
      "requiredLabel": false
    }
  }
};
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