Hello Peter @peterb
Thank you for your interest to making great translations of wpinvoicing.
We are on mission to put the all our plugins translations directly on wordpress, including wpinvoicing on https://translate.wordpress.org/locale/nl/default/wp-plugins/invoicing
Please check our our GD translator handbook for more detail here: https://wpinvoicing.com/docs/handbook/
Let me know if you have any question. You can also contact me on wordpress.slack as @ismiaini.