The dutch language file (and propably also the german and english language files) for event calendar was pretty outdated.
Here is the new file with about 35 new translationtags.
The new tags are marked with /**/ to support fast updating by other translators.
EDIT: for some reason the forum renamed the file to nlNL5.php (it should be nl_NL.php), so I uploaded the file in a zip-archive. But for some other reason, the forum didn't allow me to delete the .php anymore... So you now have 2 options: either download the raw .php-file and rename it, or download the .zip and unpack it. :P