I tried my best to simulate your situation MeruRikku and here's the results:
In the first one, i set windows to run non-unicode programs in japanese since I saw a couple of katakana characters in your screen shot and I got the same error you did. I went back and set windows to run non-unicode programs in korean and did the same thing and got a different error (because I didn't fill in the rest of the fields).
So, you can try setting windows to run non-unicode programs in korean and see if you can register (you can try applocale, but I didn't really feel like trying).
[edit]I guess you can try that too AnimePwns, since when i set windows to japanese but told Internet explorer to encode text in korean, it wasn't able to display everything in korean and displayed the rest in the native language which was japanese.
Good luck