As long as a Unicode font is installed, it should display correctly regardless of what font is selected for the window. If no Unicode fonts are installed or if mIRC can't understand the Unicode font(s) that are installed, then it will have issues displaying some Unicode characters. Also, certain fonts that you can download are poorly made and may state that they have all the Unicode characters when they really don't. mIRC may try using the font linking to link to that font because the font says it has the characters, but since it doesn't, the character won't display.
