On my website I am using K2 and of course I also donwloaded the translation in my language (dutch). On the frontend the K2 is still using the english language for the dates in the articles, categories, etc. I don't know why this is not changed into my own language.

You can look at the website at: pvt.joomla-designers.nl/k2/category-view.html

Who can help me solve this problem please?

Hello,

I have fixed this issue for a client before, unfortunately it was long time ago since I did it, so I can't remember the details.

Anyway, somewhere in the template override file, you can use this to get the month name translated into your language:

Never, EVER post credentials over the public forum unless it is a dummy registered user.

You can try re-downloading and reinstalling the language file in case something went wrong the initial file.