Hello,

For language, if you have installed iCagenda Dutch package, and you already have installed the Joomla core Dutch package, so you just have to set the Dutch language for your Joomla site in admin.

Best regards,
Cyril

The Chaser wrote: thank you, I did not know it needed the dutch core pack as well

You're welcome!

In fact, as for all extensions language pack, you need first to have the Joomla core package install for extended language files to be loaded.
The Joomla core language declares the language in the system and create needed data for it to be managed by Joomla multilanguage.

Best regards,
Cyril