Hello,
Apologies, but the Tweet Button has not yet been translated into Greek. We appear to have a glitch in the Tweet Button configurator where incomplete translations are appearing in the drop down list.
New translations for Twitter are constantly being worked on, and once they reach completion they get rolled out into the products themselves. You can find out more about the ones in progress (or even join in the translation effort) on our Translation Center site, http://translate.twitter.com.
Thanks,
Ben