xfce4 install on Ubuntu 18.04 has no polkit agent

The problem was evident as en error message when trying run 'Language Support' (gnome-language-selector) after install. (This was the advised way to get the Atom spell-checker working).

Error described here: https://askubuntu.com/questions/1031319/language-support-in-18-04-not-working-org-freedesktop-policykiterror-failed/1127862#1127862

It occurred on a fresh install of Ubuntu 18.04 after installing xfce4.

No PolicyKit Authentication Agent was installed - or at least not one that xfce seemed to recognise.

I solved the problem by installing one:

    $ sudo apt install policykit-1-gnome

That installed the package: /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1.

It seemed to update the autostart list.

Though there is such a thing as 'xfce-polkit' and 'xfce-polkit-git', I could not see them on my machine and Debian [reports][3], in it's account of the package policykit-1-gnome, that xfce still uses the gnome polkit agent:

"This implementation was originally designed for GNOME 2, but most GNOME-based desktop environments, including GNOME 3, GNOME Flashback, and MATE, have their own built-in PolicyKit agents and no longer use this one. The remaining users of this implementation are Cinnamon, XFCE and Unity."

(https://packages.debian.org/sid/policykit-1-gnome)

It is worth noting that the terminal emulator that comes default in Ubuntu and is still default after installing xfce (gnome terminal) would not run polkit-gnome-authentication-agent-1 after install. I understand it may have been necessary, perhaps ironically, to run it an an xterm. I didn't attempt this. But all was well after restart.