Well I can't say for Windows but in Linux APL is one of the keyboard layouts standard in basically any install. In KDE you can just go in and enable it, in gnome/xfce/cinnamon you have to tell the system to actually show you all the layouts
gsettings set org.gnome.libgnomekbd.desktop load-extra-items true
in a terminal will make it show up, then you just set it to toggle with some button.
When I used the Dyalog APL free trial on Windows it installed a new keyboard mapping as part of the process. I think the keymapping is also on their site. It uninstalled as well I think when I was finished and uninstalled.