That's strange. I don't have a Nexus 6, but I use Nova Launcher with no affect on Android Pay.Im in the US.
Now I seem to be able to make the app work... only when I went into the settings area with turning on android pay within the Google account area. I just can not make the app short cut on the home page, or the icon in the app drawer open the app. I tried uninstalling the app and reinstalling....but same thing.
I am also using Nova Launcher .....could that be why the app icon wont trigger the app from staring up?