Defence of the Ancients 2 is the first to get Steam's Pirate makeover.
This has been coming for a while now; I'm reliably informed (by a passing parrot) that it's one of the signs of the approaching Apocalypse. As this trailer demonstrates, rascally buccaneers have commandeered Defence of the Ancients 2 and given it a Pirate Language makeover. This is an official Steam mod, and it's only the first; Team Fortress 2 will be the next to have its timbers shivered.
You can blame - or praise, depending on your daily rum consumption - Steam's user community for this one, specifically the Steam Translation Server members. Pirate was originally a demo pack, intended to show new members what the technology could achieve. Basically a joke language, it caught on, and now is an actual honest-to-Davy Jones mod.
Warned Steam in its press release, "keep in mind that there are a few, unavoidable issues. Any web-based text can not be replaced, since it is not stored locally. Also the Pirate language pack will revert to English after any DotA 2 update. However, other than that, we consider this to be a complete build."
The language pack can be had via download here. No word on when Team Fortress 2 will get its pack, but as they've just had Mann vs Machine Steam may be waiting for that to settle in before introducing new stuff.