[Plug-in Events Calendar] switch to other language

I am using the above mentioned plug-in.
Translated the phrases in Dutch (po mo files)
Set language to nl-NL in wp-config.php

Now the options menu of the plugin is in Dutch and when I click the little calendar button next to the date field a Dutch menu pops up, but… the calendar shown in the sidebar still uses the English terms for day and month names.
How do I change that?

TIA