You could use a diff/compare function in a text editor like Notepad++.
Then open the latest English version and the latest NL version (pref. UTF8 version).
Then remove all the translations. For example:
$_lang["about_title"] = 'Over MODx';
remove the
= 'Over MODx';
If you do that for both files (which is a crappy job to do), and you compare both files, the compare functionality should highlight the differences between the files. This way you will know which language vars are missing and you can add them manually. Sorry cant think of a easier way to do this