http://www.dyalog.com/apl-font-keyboard.htm
if you have gnome or cinnamon (possibly xfce) enter
gsettings set org.gnome.libgnomekbd.desktop load-extra-items true
into a console then the APL mappings will show up in the inpute options, if you're in KDE it'll already be visible. Either way once you do that you can set it so holding some key (i use the win key) switches layouts.