Modify /etc/X11/xorg.conf (/etc/X11/XF86Config if you're running XFree86™) in the following manner:
Option "XkbLayout" "de(euro)"
Again, replace de with your language. By now, the keyboard should be set up correctly. As in the console section, the correct font must be chosen. For KDE, go to the KDE control center -> Personalization -> Country & Language -> Charset and change it to ISO8859-15. Similar steps apply to kmail and other applications.
Another good idea is to modify your fonts.alias files. Notably the fixed font should be changed to the right character set: The author's /usr/X11R6/lib/X11/fonts/misc/fonts.alias looks like this:
! $Xorg: fonts.alias,v 1.3 2000/08/21 16:42:31 coskrey Exp $ fixed -misc-fixed-medium-r-semicondensed--13-120-75-75-c-60-iso8859-15 variable -*-helvetica-bold-r-normal-*-*-120-*-*-*-*-iso8859-15 (...)
As in the console sections, special applications still have ISO8859-1 fonts configured in their respective xrdb(1) databases. One notable example is xterm. As a general rule of thumb it suffices to change the corresponding configuration file in /usr/X11R6/lib/X11/app-defaults and add the correct font. Let us demonstrate this with xterm.
# cd /usr/X11R6/lib/X11/app-defaults/ # vi XTerm
Add the following line to the beginning of the file:
*font: -misc-fixed-medium-r-normal-*-*-120-*-*-c-*-iso8859-15
Finally, restart X and make sure, fonts can be displayed by executing the above awk script. All major applications should respect the keyboard mapping and the font settings.