I choosed LXDE as desktop environment. My objective was to have the smallest and fastest desktop environment.

Install LXDE

pacman -S lxdm

Launch it

I decide not to launch it automatically at laptop boot.

So I launch it manually, when needed, via systemd:

systemctl start lxdm

Switch to a terminal

When within the LXDE desktop environment, you may leave the graphical environment and switch to up to 5 terminals by typing Ctrl-Alt-F2 to Ctrl-Alt-F5. From any of these terminals, switch back to the graphical environment by typing Ctrl-Alt-F1.

X configuration

LXDM/LXDE are based on X; a X server is ran and configured by LXDM when this latter starts. At LXDM startup, LXDM launched a script that configures the X session.
This script is /etc/lxdm/Xsession. Its content can help understand what files are read and sourced before X LXDM starts.

Keyboard configuration

X keyboard configuration is stored in /etc/X11/xorg.conf.d/00-keyboard.conf

A convenient way to edit this file is to use an executable named localectl.

If you type localectl without arg, it will output info about the locales set on your system.
When I installed Arch, configuration file /etc/X11/xorg.conf.d/00-keyboard.conf did not exist and localectl outputed this:

System Locale: LANG=fr_BE.UTF-8
    VC Keymap: be-latin1
   X11 Layout: (unset)

The 2 first lines are the consequence of my use of configuring my Belgian keyboard when I installed Arch, using this command:

loadkeys /usr/share/kbd/keymaps/i386/azerty/be-latin1.map.gz

For adding the keyboard configuration for X, type:

localectl set-x11-keymap be

Now, have a look at /etc/X11/xorg.conf.d/00-keyboard.conf: the file exists and is configured.

For finalizing locale configuration, edit /etc/locale.gen and uncomment the link corresponding to your locale, ie fr_BE...
Then, run:

locale-gen