I've edited the "danish.php" file in the "lang" folder.
The line: $LANG["FormPreview"] =".........."
I deleted everything between the " " and put in my own (danish) words. Saved and uploaded.
Apperently the "ø" for the danish "ø" was not welcome in that line.
My line now looks like:
$LANG["FormPreview"] = "Se før det gemmes";