Hi sbelaud,
The labels for your form fields are based on the names of your fields, which come from the “FIELDS” tab in the plugin admin. So, those are saved to your database. They are not strings in the plugin files, which means, naturally, that they won’t be picked up by a translation plugin that is searching plugin files, or a .pot file, for localized strings. So, what it sounds like you would want would be a tool that lets you translate strings/entries from your database, such as WPML’s String Translation tool. Perhaps Polylang offers something similar.