Thanks for your help in narrowing down this issue. I have not been able to reproduce it yet (tested under Windows 7 32bit, an English version with system locale set to Japanese, using Western/Japanese font scripts in mIRC, and /echoing and /msging myself with the combination of characters you mentioned).
Could you try one more thing: could you try toggling the switches in the Options/IRC/Messages dialog "SJIS/JIS conversion", "Multibyte display" and "UTF-8 display" to see which one is the cause of the crash? That would help narrow it down even further.
Also, if you use the "/debug on" command it will output raw server text to a debug.log file. When mIRC crashes, the last line or two in the debug.log file will likely be the cause of the crash. That would be of immense help as well.