SJIS/JIS conversion is currently a global option in IRC->Messages.

I'm on channels that uses different encodings
* UTF-8
* ISO-8859-1(5) (latin1, latin9)

If I have SJIS/JIS turned on, mIRC encodes some ä* and ö* combinations as JIS, for example
* ää -> $Bhf(B
* äö -> $Bhx(B
* än -> $BgO(B

Without SJIS/JIS I can't chat on Japanese channels.

That's why I really think SJIS/JIS conversion should also be a per channel/query/window option like UTF-8.