I have currently a model with properties defined, and I use those properties in de Fieldset class to generate a form. But how can I define translations for those model properties?
You mean, for the 'label' fields?
The Form class instance will automatically do a \Lang::get() on the defined label, so if a language string exists that matches the defined label, it will load the translation.
I code my labels as "label.model.column", which translate to array('label' => array('model' => array('column' => 'label-for-column-name'))); in your lang file.
This allows me to have per-model language files. I do a \Lang::load() in the model _init() method, so they are loaded before they are needed, and they are loaded only once.