If decimal type questions are used, the calculation field in Survey123 Connect for ArcGIS on a French operating system returns the error message, "Invalid Number."
It is not the decimal symbol that is causing the problem here but the digit grouping symbol. It is set as ' ' (space) which would not work with a decimal (.) while using the app in the FR locale.
Please try using a decimal (,) with grouping symbol (' ') in the Windows Regional Settings and don't forget to restart Survey123 Connect for this to take effect. This combination is accepted when using the app in the FR locale.