For installation instructions please see the official help article.
In order to build the plugin you can run the following command:
make version=4.0.00
make clean
This will create newsletter2go_v4.0.00.zip which you can then upload to your WordPress installation.
Currently the plugin is deployed by uploading the built package to our website.
Translations are located in \wp-content\plugins\newsletter2go\lang\ directory.
Use .pot file as template, it has all translation strings set.
Create .po file for new language. It must be named in format [plugin-name]-[locale].po (eg newsletter2go-de_DE.po). Locale code is according to WordPress coding standard (https://wpcentral.io/internationalization/).
When translation is finished, file .mo should be compiled from .po source. File .mo is machine readable format that WP system uses. At production location, the .mo file is only necessary.
For generating .mo files, translating and creating .po files as well, Poedit software could be used (https://poedit.net/).