That error is logged:

org.gnome.ScreenSaver[1081]: Unable to init server: Could not connect: Connection refused

then:

gnome-session[895]: gnome-session-binary[895]: CRITICAL: Unable to create a DBus proxy for GnomeScreensaver: Error calling StartServiceByName for org.gnome.ScreenSaver: GDBus.Error:org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.gnome.ScreenSaver exited with status 1

oem@ubuntu:~$ localectl
   System Locale: LANG=en_US.UTF-8
       VC Keymap: n/a
      X11 Layout: fr
       X11 Model: pc105
     X11 Variant: oss

So, with googling help (https://bbs.archlinux.org/viewtopic.php?id=180103), i've edited /etc/locale-gen (as user) and uncommented the line: fr_FR.UTF-8
 and then ran 'sudo locale-gen' and finally rebooted.

But the error is still logged after reboot: either i need to set also 'VC Keymap', but how ?, or it is related to the vte version actually used (and need an upgrade) (https://bugs.launchpad.net/ubuntu/+source/vte2.91/+bug/1721412)

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-session 3.26.1-0ubuntu7
ProcVersionSignature: Ubuntu 4.13.0-18.21-generic 4.13.13
Uname: Linux 4.13.0-18-generic x86_64
ApportVersion: 2.20.8-0ubuntu2
Architecture: amd64
CurrentDesktop: GNOME
Date: Sun Dec 3 17:17:45 2017
EcryptfsInUse: Yes
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-session
UpgradeStatus: No upgrade log present (probably fresh install)