Thanks for the VM, it helped me realize what I was doing wrong and I was able to reproduce the error on my box.
The problem was that when CloseXML opened a file, it picked up the numbers without processing them (in a culture invariant format). Later on when you saved the file it assumed the numbers were in the culture of the machine.
I really wonder why our German friends didn't pick this one up before...
Anyway, it's now fixed on v0.42.1 (it now loads numbers into the culture of the machine)
Thanks again for your help,