the problem I'm having is that cyrillic text looks bold (Example) even though other languages in the same places are OK. What could be the reason for this behavior? Could it be that the font used doesn't have those characters and some substitute font is used? Or is it because of wrong charset or something (sorry I'm a total noob when it comes to encodings etc...). Thanks for any help.
Ok, your URL helps heaps.
You are using the Ubuntu font from Google Font API: http://www.google.com/webfonts#UsePlace:use/Collection:Ubuntu
You need to ensure you're using the Cyrillic and / or Cyrillic-extended subset in your font.
Go the the font page again and select the correct checkboxes before you copy the script to include in your pages.
Check for the URL to the CSS to look like this:
<link href='http://fonts.googleapis.com/css?family=Ubuntu:300,400,500,700,400italic&subset=latin,latin-ext,cyrillic' rel='stylesheet' type='text/css'>