YouTube didn't translate the titles and descriptions for all the videos, but it decided to use Google Translate's JavaScript API to perform the translations when they are needed, much like Twitter Search. If you click on "translate results into my language", YouTube sets a preference and all the search results will have titles and descriptions in your language, assuming it's supported by Google Translate.
The translation API offered by Google can be easily integrated in web applications, especially that it can now preserve HTML markup. I expect to see a lot of applications that take advantage of the API, from webmail services like Gmail to browsers and IM clients.
In other machine translation news, Google Dictionary has a new interface and it now supports more language pairs. There's also a section for definitions found on the web.
{ Thanks, Aleksandr. }