Andrew,
Sorry for the delay. First off, thanks so much for contributing the Dutch translations! You can install those .ini files by simply copying them to your /languages/nl_NL/ directory. As long as that's the setting for your Joomla site, JFBConnect will automatically start using them.
Additionally, we've included your files in the v5.0 release that is coming out later today. Once you install the new release, the new files will be there automatically and should be used right away.
Hope that helps, and thanks again for your support,
Alex