[Plugin: Twitter for WordPress] Months not localized

I have some strange behavior with this twitter plugin on this website I’m testing (here, in the footer is the twitter plugin).

I am working with the dutch version of WordPress, and have set the settings to dutch (timezone etc).

All localication functions in this plugin work properly, such as when the tweet is posted less than 24 hours ago, the sentence “7 hours ago” is translated perfectly. All work, except for one; the months keeps displaying in English! The order of the day, month and year is displayed in the right (dutch) order, and my language files (nl_NL.po and -.mo) contain all the strings of the english and dutch months.

I’ve been trying to tweak the following code of the plugin, but to no avail. Does anyone have a clue?

if($update) {
          $time = strtotime($message['pubdate']);

          if ( ( abs( time() - $time) ) < 86400 )
            $h_time = sprintf( __('%s ago'), human_time_diff( $time ) );
          else
            $h_time = date(__('Y/m/d'), $time);

          echo sprintf( __('%s', 'twitter-for-wordpress'),' <span class="twitter-timestamp"><abbr title="' . date(__('Y/m/d H:i:s'), $time) . '">' . $h_time . '</abbr></span>' );
         }

http://wordpress.org/extend/plugins/twitter-for-wordpress/