Hey Peter,
I have checked the issue and it seems that this is related to your other installation issue where the template folder cannot be found when there's no default template for all pages. I have already submitted a report for the devs to look into it.
In the meantime, the best way to temporary solve this is to set a default template for ALL pages(
http://take.ms/DdYaX).
You can duplicate a G5_hydrogen template, set it up and make it default for ALL pages, while keeping the other 2 default language templates.
After that, try and see if the template override works.