General #
The easiest way to control the keyboard layout is localectl, because
that is the one-tool-for-all-solution.
localectl list-keymaps
localectl set-x11-keymap de_DE
localectl --help 
However if you haven't that installed or available on your OS, you can probably find the available keyboard layouts in the manual:
man xkeyboard-config
X-Server #
available layouts, variants and models can be found with
setxkbmap de               ;# short form
setxkbmap -variant intl us ;# with intl variant (more keys with right alt)
setxkbmap -layout de,us    ;# long form with two layouts
Variants #
grep -E ^xkb_symbols < /usr/share/X11/xkb/symbols/us
setxkbmap us -variant intl
setxkbmap us -variant colemak
grep -E ^xkb_symbols < /usr/share/X11/xkb/symbols/de
setxkbmap de -variant neo
Options #
reset keyboard options (like caps:none or caps:swapescape)
setxkbmap -query      ;# displays options
setxkbmap -option ''  ;# removes options
man setxkbmap says: “If you want to replace all previously specified
options, use the -option flag with an empty argument first.”
Autorepeat when holding keys #
This makes backspace and the cursor keys more enjoyable and is called
Typematic Rate Setting in some BIOS:
xset r off          ;# disable autorepeat
xset r on           ;# default setting
xset r rate 330 40  ;# explicitly set new rate and delay
Apply changes permanently #
# /etc/X11/xorg.conf.d/10-keyboard.conf
Section "InputClass"
    Identifier "Keyboard Defaults"
    MatchIsKeyboard "yes"
    Option  "XkbLayout" "us"
    Option  "XkbVariant" "altgr-intl"
    Option  "AutoRepeat" "330 40"
EndSection
Terminal #
Real terminal users need another toolchain.
loadkeys de
Configuration files #
To make changes permanent (surviving a reboot):
/etc/locale.conf   # @see: man locale.conf
/etc/vconsole.conf # @see: man vconsole.conf