As far as I know, UTF-8 can be enabled for all windows by going to the Font options dialog and checking "Use as default". I don't think this option appears for all windows, but I've been able to enable UTF-8 globally using that.
Yeah, you're right, i realize that after posting, but forget to edit, heh.
I'm not sure if that's a feature Windows even has.
I don't know about Windows, but i'm sure what mIRC have, because this is already realized in Editbox.
Here is the confirmation:
1. Select the default mIRC font, Fixedsys, what doesn't have diacrtics for Western Europe languages.
2. Activate Options->Irc->Messages->Multibyte Editbox (that would activate "substitute missing characters")
3. Write in a editbox, some special chars, what Fixedsys doesn't have.
Notice the underlined with red chars, these was taked from another font.
I suggest to implement same think for channel/private messages.
Current, mIRC don't try to make anythink(at least to show a err message, maybe ? with a suggestion to change the font), but show a odd char(see these underlined with blue).
Imho, in current situation, when the default font is Fixedsys, what doesn't have any support for unicode, utf-8 power can't be used by simple users what doesn't know what he must change the font to see the special chars..
mIRC must use a unicode supporting font by default, or at least substitue these special chars from other fonts..