– make sure you have the English language installed check language / your default language set to English US / check options, ‘windows display language’ states should be enabled.(if not select the download option)
– in HKey_Local_Machine/System/CurrentControlSet/Control/Nls/Language change the string “InstallLanguage” to 409
– restart your Surface, and retry the update for http://preview.windows.com
The update will now install and after reboot you’ll be notified to update to 8.1 via the Store.
Mind this does take a while, but it’s worth the wait 🙂