Hi @jobsworth, your post I have moved from 1st Impressions. It does match this topic, doesn't it?
That was my experience, too. After getting rid of all Gnome residuals it was no problem installing xfce4. To get a graphical login page again, only slim needed to be installed. No tweaking was necessary and the desktop environment is started without any detour.
The old users of my Debian Wheezy were all present after the upgrade - their data included. Even if your xfce environment is someway infected, the 'adduser' command should be working.
Using mainly my clean installed Devuan Jessie with Mate desktop environment, I don't have a lot of experience using the xfce environment.
Sorry to hear that. Unfortunately the installer assumes some kind of connection between its language and the location / time zone of the system. You can take a look at the man page of 'timezone' for information. In my case I encounter problems only because of a multi boot system.
Glad to hear you are not disgruntled by all these obstacles!