Created by a group of hardy volunteers on the Steam Translation Server (a group of volunteers that do translations for Valve games) this is clearly a labour of Piratey love.
Initially the whole Pirate thing was supposed to work as a demonstration, to teach people how to use the Translation Server, but people continued working on it, and before long it was a fully fledged project. This is the result. DOTA 2 making every single day Talk Like A Pirate Day.
You can download the language pack here.