Hi UCLA-Under,
I understand you're having difficulties setting the language on your Officejet 8600 to Korean. I'm sorry to hear this has been happening but will certainly do my best to help you.
Unfortunately, as ShlomiL stated in the post above yours, you cannot roll back the firmware. Once it has been updated there is no going back, so I decided to test a few things on our Officejet Pro 8600 that we have here. It had an older version of firmware, and had Korean as a language option, so I updated the firmware to the latest version. After the update I checked the languages again and I still had the Korean language option. As far as I could gather from my research the Korean language should be the option that looks similar to this: 한국의
If you still aren't seeing this option could I get you to tell me what version of firmware you have? To check this have the printer on the home screen, press the white ? in the top right corner and click Printer Information, and the FW Version should be the last option.
Hope this helps, have a great day!!