我的英语水平不是太好，对你所问的问题都不是很明白

It’s not working on this platform since you’re only able to switch from two langages as default setting is compeletly lock on(with site code and those stuffs) so means you have to find another sites which provides korean as an defult langauge settin, hope this helps.

Maybe you can try to play with English version of YGOPro? I guess reading English would be much better for you than reading Chinese.

First, confirm that your font exists in that directory.
Second, MyCard client would overwrite the system.conf file, for setting the username and e. That could be a reason why it always turns back to default. So font changing may not be working.