ISO-8859-15 is missing from the menu in "File->Server List... -> (select an ircnet) -> edit -> Character Set"
It took me ages to find out that you can do a "/charset ISO-8859-15" on the irc :)
The whole thing popped up just after I upgraded to Tiger. In Tiger I couldn't see any umlauts on some channels. In Panther everything was fine. I'm using the same version of X-Chat Aqua as before (0.14).
For now, I wrote the "ISO-8859-15" on the "Character Set" field, but I'm not reconnecting to see if that works (lazy). I'll see that after next reboot (Can take some time).