I was able to add additional keyboard layout in
Settings -> Regional & Language -> Input Sources
Bulgarian (new phonetic) to be specific. SuperSpace works fine, the Gnome icon also works fine for switching. The thing is that the layout is the same for English (US) and Bulgarian (new phonetic). Nothing changes if I try to switch, English (US) in both cases.
Any thoughts on this?
I've googled for about 20 minutes, the problems I've found are mostly not able to Super etc. and Gnome icons not working.