All messages and labels are directly translatable from the administration panels of Newsletter. You can find them under the subscription panel and the form fields panel.
Translations are initialized on first install with your language, if available, and then you can change them.
Newsletter is a communication tool, hence we want to be sure you can change every single word to best match your marketing efforts: this is why we do not adhere strictly to the standard way of plugin translations (using the .po/.mo files).
Those kind of translations regard only the messages and labels shown to your readers.
The administration side translation uses the translate.wordpress.org project. Anyone can contribute and become a polyglot! Translations are reviewed by editors and when published thay are downloaded and updated automatically.
Since the plugin contains a lot of text we’re constantly working to simplify the translator works, using fewer words and shorter phrases.