I have the same problem and use font DejaVu Sans.
Eg, i can't see the korean characters in this post: https://bugs.launchpad.net/widelands-website/+bug/1614403/comments/66
In firefox (sorry) this works fine with DejaVu Sans. There is also a function called "Show page properties" (or similar) which shows me that the page of the link is encoded in utf-8. Is there a similar function in opera?