I could easily be wrong here, but I am going by imperfect human memory as well as remembering some tech advice that is now several versions old.
Try saving a clone of the file and then importing all your data into the clone. That might make a difference here.
Sorry, that didn´t help either.
There seems to be something in the file itself, becuase, I can see that it change the names when I open the files, if I choose to show the calculation: DayName (Get ( AktuelltDatum )) I see that the result change from "Söndag" to "Søndag" -> That is sunday.
I have erased all data in the file, and tried to start over, but that didn´t help.
If I switch to use and don´t use system settings, it affect they way the date format is shown, but it doesn´t change the dayname.
This stupid problem is still unsolved as of Filemaker 15.
The DayName() function will always return its result in the language used when the database was created.
If you create a database on a german system, Dayname() will forever return Montag instead of Monday.
If you want to change this behavior, you can make a clone. But now you're locked into getting Monday forever.
This does absolutely not help in a country like Switzerland, where we use at least 3 languages.
The only solution is to avoid using DayName and roll your own custom function.