Hi Onno,
In order for you to activate the language, you will need to ensure that the language pack is installed on your Joomla site.To install a language pack for Joomla, navigate to Extensions > Language Manager. Then, click on the Install button.

After that, head to your EasyBlog's backend Language page and look for the language(http://take.ms/FvC5G). Select it then click on install. Remember to set the language as the default language in Joomla if you want to make it default language.