It is probably not a browser problem, and also not an SE problem. It is a problem probably in your local network. Your computer somehow doesn't get the IP address of the site from which the MathJax is downloaded from. If you are at home, restart your router. Alternatively, there are also alternate solutions, they are more tricky, you can also manually insert its ip address into the
C:\Windows\System32\drivers\etc\hosts file (in windows). Probably it is solved until now.
About the Chinese text in your this file: In this file, there shouldn't be any Chinese, only English, even on non-English Windowses. Probably you have a virus. Trashing the hosts file was only a side-effect of the infection. Alternatively, it is also possible that some file corruption happened on your machine on unknown reasons (typically, sudden poweroff causes it).
As you write, replacing the hosts file from here solved the problem.