Marc,
Whatever you are writing has nothing to do with excel, so you should not call the file *.xls, which is a proprietary microsoft file format. Write your file e.g. as *.txt and look at it in notepad. It will look fine.
By naming your file *.xls, you artificially force it to be opened with excel and excel must try to guess how to read and convert it. It will make mistakes.
As far as I can tell, (at least on a US computer), the comma is treated as "1000" separator. In the US, your LabVIEW program will uses a "." instead of a "," as decimal separator and everything works correctly.
I would recommend to disable the localized decimal point in LabVIEW. (uncheck: "Tools..options..Front panel: Use localized decimal point"). In addition, you should also structure the output a bit better. Get rid of the spaces in the format string so the numbers are cleanly separated only by tabs.
(Sorry, I cannot test in more details because I only have a US computer).