Change main keyboard layout in OS X
When I tried enabling the setting (Input source options: Allow a different one for each document), new windows seemed to just use the current input source.

Anyway, you could try running sudo /System/Library/CoreServices/Setup\ Assistant.app/Contents/MacOS/Setup\ Assistant and selecting US as the default input method. (You have to go through the steps of creating a new account, but you can delete it later.) It modifies at least /var/root/Library/Preferences/ByHost/com.apple.HIToolbox.*.plist.

If that doesn't work, try running sudo languagesetup and selecting English, or sudo defaults write /Library/Preferences/.GlobalPreferences.plist AppleLocale en_US and restarting.