How to translate fields added dynamically to a Model (or CMS Page)

Since the RainLab.Translate plugin uses the backend.form.extendFieldsBefore event to replace regular form fields with their multi-lang equivalent, new fields must be added using this event as well, otherwise they won't be translatable.

In your plugin's boot() method, create the new field and add it to the model's translatable property array:

Event::listen('backend.form.extendFieldsBefore', function($widget) {
   $widget->tabs['fields']['myField'] = [
      'label' => 'My New Field',
      'type' => text,
      'tab' => 'myTab'
   ];
});

YourModel::extend(function($model) {
   if (!$model->propertyExists('translatable')) {
      $model->addDynamicProperty('translatable', []);
   }

   $model->translatable = array_merge($model->translatable, ['myField']);
     if (!$model->isClassExtendedWith('RainLab.Translate.Behaviors.TranslatableModel')) {
        $model->extendClassWith('RainLab.Translate.Behaviors.TranslatableModel');
   }
});