I have installed https://launchpad.net/ubuntu/xenial/+source/fonts-noto-cjk, but still has the same problem in Chinese websites. Then I uninstalled fonts-noto-cjk from Synaptic, Chinese characters of Chinese websites are back. Surprise!!! I don't know what happen, but I hope this comment can help you.

Opera 39 on Ubuntu 16.04 64-bit