Guillaume,
The forum software ate the part of your question where you told us you were using Write to Spreadsheet.vi, because it assumes anything within angle brackets is to be treated as HTML. But the magic of "View Source" made things clear.
This VI takes an optional format input that will solve your problem. What you want to do is prepend the string "%.;" to a normal format string and use that as your input. For example, "%.;%.3f" would force a period separator and give you the standard three digits after the separator in your spreadsheet file (see attached).
You can find documentation for this if you dig all the way down to the "Format Specifiers Syntax Elements" help topic. I'm assuming you also know about telling LabVIEW not to use the system's localized separator (Tools >> Options >> Front Panel), if you want to override the behavior globally.
Regards,
John
