I use Loco Translate. It reports that several files are missing:
Plugin has a strange POT file name (it_IT.po). A better name would be “newsletter.pot”
Most end user (newsletter subscribers) strings can be put in the admin forms of the plugin. Not all, because when generating an email from posts, many of the strings in there are in English.
If it would be possible to translate all strings, you can start with a default in your own language. Being able to override the default in the admin UI, would still be useful in case you want your messages customized. But it would save a lot of work to set it up initially.