@toerfiets: Our themes are translation ready but Dutch Translation file is not available for Lucida Pro Theme. To translate those texts, you will need to add Dutch language file nl_NL.po which you can get by editing the lucida-pro.pot file in languages folder.
What you need to do is Open that file from free software from http://poedit.net/ then create New Translation and select the language Dutch(Netherlands). Now search for words like previous, next and the words you want to translate and add the translation for those words and save it. You can also add other translations there as well. The words will be translated when you change your site language.
You can send us the translated .po file and we will add it in our theme with you as a contributor for the translation.
Let me know if this solves your issue!
Kind Regards,
Skandha