Really Simple Twitter Feed Widget: timestamp in different language

In my twitter feed, the timestamp shows as ’22 May 2012′
I would like this to be in Dutch ’22 mei 2012′. How can I change this?
My locale is set in wp_config as define('WPLANG', 'nl_NL');

I tried adding setlocale(LC_ALL, "nl_NL"); to the plugin code (really_simple_twitter_widget.php) but this has no effect.

I’m using WordPress 3.3.2 and Really Simple Twitter Feed Widget 1.3.7

Thanks for any help!

https://www.ads-software.com/extend/plugins/really-simple-twitter-feed-widget/