By becoming the System Settings, the System Preferences lost three functions: the configuration of the network connection priority order, the start-up and shutdown programming interface, and the customization of some language preferences. But Excel used these preferences to choose the decimal separator and the thousands. This forces Microsoft to finally add an option in the settings of its spreadsheet.
To customize the separators, open the section Modification Microsoft Excel settings, then deactivate the option Use system separators. You can then choose your own decimal separator, a comma by default, and your own thousands separator, a space by default. The Windows version of the spreadsheet had this option for a long time, but the Mac version relied on system settings.
Unfortunately, this option does not appear (yet?) in version 16.66.1 distributed to owners of a “perpetual” license, but is available in version 2210 distributed to subscribers to the Microsoft 365 offer. Both versions have always a few weeks off, and some functions are sometimes reserved for the Microsoft 365 edition.
To be completely complete on the issue of language preferences, note that the options in the old section Keyboard have been distributed between the new section General > Language and region (where you can now easily configure the language used by an application) and the new section Keyboard (especially in the section All input methods who hides behind the button Modifier…). This new organization breaks the benchmarks, but is ultimately very logical.