The contents of /etc/default/locale would have been valuable...
Anyway, this seems to be an installer issue rather than something that affects lightdm. I suppose that "xubuntu" is a special purpose user for demo purposes. Possibly it's something with that user that prevents accountsservice from providing the correct default language. I can't tell if it's a Xubuntu specific problem or if it's inherited from the Ubuntu image. Changing affected packages.
The contents of /etc/default/locale would have been valuable...
Anyway, this seems to be an installer issue rather than something that affects lightdm. I suppose that "xubuntu" is a special purpose user for demo purposes. Possibly it's something with that user that prevents accountsservice from providing the correct default language. I can't tell if it's a Xubuntu specific problem or if it's inherited from the Ubuntu image. Changing affected packages.