System font corrupted after trying to install KDE

[axioma]

Today I tried to install Kwin from the repositories because of screen tearing in Xfce. Some how Kwin did not work well so I decided to install the KDE plasma desktop. The install went well but after booting into it, my system fonts were screwed-up.
When logging out and logging into xfce, this did not change. I hoped that by completely removing the KDE desktop this might solve the problem but the font problem had not changed. I tried to change the font settings in the settings manager but this also had no effect. I even removed /.conf/ and /.cache/ in home, but this only reset the desktop but not the font. It seems that only these programs that depend on the system font settings are affected. Those that have their own settings like browser, etc. not.

I wanted to add a screen shot but could not upload it. Therefore a link to Google drive with the screen shot. Anyhow, I suspect somewhere a file has to be edited but I do not know where to look.
https://drive.google.com/file/d/0B1E-kzA1HrhvWXctLUdmZGxrN1E/view?usp=sharing

[paxmark1]

You do know that now is the libstc++6 transition AND the kf5 transition.  https://release.debian.org/transitions/html/libstdc++6.html    lurk on #debian-kde. 

Every other flavour of sparky is going to be easier than KDE at this time.  It has been a very bumpy ride for upstream unstable and it is now hitting testing  for KDE.  If you are brave and knowledgeable, carry on.  Keep  xfce as a backup for logging in (are you lightdm or sddm?)  Eventually KDE5 will settle down but as you can see from the debian /html/libstdc++6 page there are dozens and dozens of packages transitions from kde at this moment.