I believe you can change it in your ACP > Skins > YOURSKIN > Language section. Switch the language drop down to the language you want to use and click the Save Settings button at the bottom of the page.
The Elastic skin is setup for the new Site Builder as well as the Lucid skin.
The only item you can upload through the Site Builder module would be images, everything else, videos etc... needs to be uploaded via your profile as usual, then you can embed them via the Site Builder.