This plugin implements a Flattr shortcode. It comes with its own options page under the Settings menu and has been translated into French.
I am learning about WordPress plugins and how to develop them. Flattr is an excellent idea and so...this plugin was born.
A future version could include Twitter-integration, to notify your followers when you Flattr a thing. Being able to receive events from the Flattr frame would make that possible, but using the upcoming REST API might work better.