I got it to work myself today, I am not 100% sure if all the steps I did are needed, but do them just in case!
1) Uninstall the Create-A-Pet version you have, it will go to the uninstall programs window from Windows.
2) Go to www.ea.com and log in at the right and go to your account (you have to click your username to get there).
3) On the first tab, change your country and language settings to an English speaking country. I chose UK and English (UK)
4) In Origin re-download the CAP Demo,make sure to choose the English (US) Eula and agree to it.
5) The Demo will install itself, at some point you get a popup telling you are ready to play it.
After all that is done, run the demo and you will see its in English!