I’m on win7 and ctrl+з works the same as ctrl+z.
I haven’t tried it on opensuse, it could be one of those kde/qt keyboard issues linked to missing xlib functions, kind of like the unicode one that X is supposed to provide but does not. https://bugs.freedesktop.org/show_bug.cgi?id=26747
At the moment I don’t have any gtk2 or gtk3 DE’s installed so can’t test it there but it probobly works with lxde/mate/cinnamon/gnome.
Have checked the KDE Bulgarian keyboard layout and, as you say, none of the Alt keyboard options are used. It might be worth putting in a bug request to KDE asking them to add the US keyboard layout to the Alt options.
did some more tests on opensuse 13.2 x64 kde 4.14.9
цтрл+з works for me
I tested it in libreoffice writer a non kde app, in firefox a gtk2 app, and in kate using a cyrilic keyboard layout the shortcut ctrl+з works, it even works here in this comment box.
How did you setup the keyboard layout what DE are you using, I haven’t installed kde5 can’t say if it works there.
about kde bugs they do have a bugzulla https://bugs.kde.org/
ps I didn’t use the bulgarian layout, could be something bg related, I tested 2 different cyr layouts no problems for me.