Switching keyboard from French to US was easy (one just need to change the layout in Settings->Keyboard) save for one tiny, tiny little issue — frankly, an absolutely insignificant detail: LightDM, Manjaro’s logon screen mind you, persisted in forcing me to use a French keyboard to login. The bastard.

Solution? Open /etc/X11/xorg.conf.d/XX-keyboard.conf (replace XX by whatever number your file is using) and manually change its InputClass to something like this:

Section "InputClass"
Identifier "keyboard"
MatchIsKeyboard "yes"
Option "XkbLayout" "us"
Option "XkbVariant" ""
EndSection

Works for me.