I have a smallish issue with Qt5 applications on my Gnome desktop (although I think the problem was present when I tried Plasma as well). Korean characters don't render properly, instead they simply appear as square boxes. This appears to be the case regardless of the font I configure Qt to use.
I'm assuming this may be a locale issue, but not sure how to work around it. The problem is not present in GTK applications.
$ locale -a C en_GB.utf8 en_US.utf8 POSIX sv_SE.utf8