What other languages are available for use at the moment and how can I change the language?
Also, currently I’m helping to create Dutch translations on: https://translate.wordpress.org/projects/wp-plugins/erp
when will the Dutch language be included in the plugin?
This doc will help you in translating WP ERP: https://wperp.com/docs/erp-core/how-to-translate-wp-erp-plugin/
And after your translated file is approved by WordPress admin then you will be able to download and copy the .po and .mo into plugins language folder (or follow this documentation) to change the language.