Thank you DiSlord for your answer.
You are right. If I change this locale setting in Win7/10, it works correctly now. I don't know why the other applications such as Saver, Partner or Sharp are not sensitive to this setting, but the main thing is that it works ! I have also discovered that the decimal separator used by NanoVNA-App is depicted on the left lower part of the main window.
I have also transposed this to the Wine/Ubuntu environment through Winetricks and the registry editor (regedit) :
Path : HKEY_CURRENT_USER\Control Panel\International
Entry : sDecimal and change "," by "."
Once again thank you for your help, and congratulations also to OneOfEleven for this master piece of software.
Jean-Roger
