I am glad webfonts worked for plm, that is usually all that is needed - a good font files, with the needed encoding.
Try "xfontsel" Play with the last 2 menus - rgstry and encdng. The combinations that produce cyrillic are either iso8859 - 5 or koi8 - r. (koi8-r - old soviet\russian cyrillic layout. It is sometimes easier to make cyrillic work on unix with that, than other encodings.)
When you select one of the two combinations, see if the sample text shows any cyrillic in it. Also check what fonts provide those characters by going to fndry menu.
Slack's major problem is that it doesn't have true, good cp-1251 ttf fonts. The unicode layer has to construct it from bits and pieces of koi8 (from misc fonts) and some other crap. That is where it fails often - you get boxes with numbers.
I didn't try webfonts yet, but if it, of any other external ttf file, contains true cp1251 encoding your comp wouldn't have so many probs rendering cyrillic at cp1251.
Try getting other fonts from windwows (preferably 2000, XP and up), like arial, times, courier. They have true unicode fonts.
Once installed, check in xfontsel again. If you see them there, but not in KDE, we need to look at KDE font "filter."
BTW: with all that hoopla about kewl, new Bitsream Verona - where the hell is the cyrillics?
You can get lots of fonts from here
Warning: the sire is in Russian only and crashes Konqueror.