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.
You wouldn't happen to know why the two layouts, besides UK and US, are Danish and Finnish? As a Dane it's kinda cool, but hardly the first language you'd expect to see listed for something this niche.
APL has historically had relatively high market penetration in Scandinavia and Japan. Dyalog is a British company but for the last decade there has been significant Danish and Italian involvement in the form of major clients and the management team who engineered a "Management Buy In" together in 2005. Our headquarters and more than half the employees are in Basingstoke, UK. We have team members in Denmark (including the CEO and CXO), France, the USA and Canada. Since January, the CTO job is back in English hands.
if you have gnome or cinnamon (possibly xfce) enter
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.