Date is all messed up in comment section using Dutch locale!

Using the Dutch locale for two sites, I came around a very strange error: when people leave a comment, the date of the comment looks like a random string, but the time is printed correctly.
First I thought it had to do with re-CAPTCHA, but disabling didn't solve the problem.
Then I thought that there must be something wrong with the Dutch locale, the nl_NL.po file for the theme, but after checking every line of that translation I couldn't find any anomalies.

And still, the date looks messed up. Has anyone a bright idea where I should look to fix it? I attached an image showing how it looks like right now on www.ingeloots.nl

#edit - I tried also switching themes: when I activate iNove or Pixel, the problem disappears, to re-appear if I activate Atahualpa again.