Fresh install of today's build of 2.0, upon the choice of Lang to use during install, I changed to en_us and received an error message:

Unfortunately the language "en_us" could not be downloaded. The installation process will continue in English.
Download of component to your server failed, please verify proxy settings, PHP cURL extension is highly recommended.

You must download the http://download.moodle.org/lang20/en_us.zip file manually, copy it to "/var/moodledata-20-dev/lang" in your server and unzip it there.

I tried to manually download from that link and it found nothing, so it's possible that's what caused the error in the first place.