[Plugin: MailChimp List Subscribe Form] Wrong language in the Signup widget!

Hi,
The Mailchimp Support team referred me to this forum.

I have setup the website https://www.feestdagen-belgie.be/ and I use WordPress V2.9.2 and the latest version of the WordPress Mailchimp plugin.
The WordPress is setup for the Dutch language [ define (‘WPLANG’, ‘nl_NL’); ] and for example the admin section is shown in Dutch nicely.

But I have noticed that the messages in the sidebar widget are shown in English, and not in Dutch as I want it.
Notice the messages “required field” and “Success, you’ve been signed up! Please look for our confirmation email!”
This is a screenshot.

So somehow the messages in the sidebar are still in English.
Please advise how to display those messages in Dutch please.

Thanks for your help!