There is a string in your language file that is used by FCK editor to figure out what language to show based on the selected language in our application.
In order to fix this, you will have to edit your German language file, and to modify the following line:
"en" = ""
to look like:
"en" = "de"
That string was added on 2007-11-13, and it should be in your language file around the line number 2211.
