Edit the keyboard configuration file at /etc/default/keyboard to the desired language. The following example sets the keyboard layout to german:
# nano /etc/default/keyboard
# KEYBOARD CONFIGURATION FILE
# Consult the keyboard(5) manual page
XKBMODEL="pc105"
XKBLAYOUT="de"
XKBVARIANT=""
XKBOPTIONS=""
BACKSPACE="guess"
To apply the changed settings in the keyboard configuration file, run the following command:
# setupcon
This may take a while. After that the configured keyboard layout will be activated.