When you go into the application settings you'll notice that it's impossible to activate Hotwords. Changing your language in the application won't make a difference. However, go to
iOS Settings -> General -> International and update your language to plain 'English'. Once the phone has finished applying it head to the Google app and Hotwords should now be on by default.
I haven't found any issues with this so far but please be aware that I accept no responsibility for any damage or other inconvenience caused to your device or yourself through following this tip. I'm offering it simply to let you know how I got this working on my iPhone 5.