All Manjaro ISO except Architect uses the locale you select in the boot menu.
If you are using a live ISO with a desktop - select your keyboard using the boot menu and when booted - open a terminal using the desktop's menu.
Using a terminal - change your active user from manjaro to root. When you are prompted for password enter manjaro and press Enter.
$ su -l root Password: #
The prompt has now changed to the root prompt and you have unhindered access to the system.
If you are using Architect ISO just select the Boot entry and press Enter to load the Architect system. Login as root with the password manjaro.
To set the keyboard layout to be used during installation, use loadkeys followed by a country code e.g. for Denmark the command is
# loadkeys dk
The available keymaps can be found by looking in the /usr/share/kbd/keymaps/ folder.
Also ensure your system clock is set properly
# hwclock --systohc --utc # systemctl enable --now systemd-timesyncd