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.
I just would like to mention that I'm facing the same issue and it would be great if FPD would cooperate with loco translate correctly. Each time there's an update I need to go a special, time consuming way in order to translate the content so I often leave it as it is. I count 26 Wordpress and WooCommerce Plugins on my site and FPD is the only one having troubles with loco translate.
So would be really appreciated if you could take this into consideration for the future. Thank you.
You could head on over to the feature request forum and have a look at the discussions there or add a new topic with a request for the feature you are looking for so we can keep it in mind for further updates.