My mother uses Xubuntu, i.e., she uses Xfce. She uses a German keyboard, but wants to write e-mails with spanish accents, i.e., she needs dead keys. At first I tried to solve the problem with a change in xorg.conf, which did not work. Some Google search revealed that a couple of people had the same problem and proposed various hints for a solution. The easiest solution is, in effect, very simple: Just comment out
If one does not want to change the key settings permanently one can define a compose key, e.g., the left Windows keys, to construct spanish or french accented characters with multiple key strokes, see Xfce forum: How to set XFCE4 compose key?.
setxkbmap -option compose:lwin
This can be put into
See Linux Compose Key Sequences for a list of compose key sequences.