I don't quite get how to setup the package, so when I translate my changes are not affecting the plugin.
basically I want to do another package like the german one.
I've never tried the loco translater plugin. If you do not have WPML I'd use the free program https://poedit.net/ to create the necessary .mo and .po files. These you'll then need to place into this folder: \wp-content\plugins\fancy-product-designer\languages\ . You can use a copy of the default.po to base your translation on.