What it is surprising is that Google Sheets does not save the file with the current settings as LibreOffice does, if that is the standard. Or it is the opposite? I tried to find this in the ODS file format specs but I couldn't find any reference about it.

Changing the localization of my LibreOffice setup before saving the document is a workaround I'm not sure to take as normal. Google Sheets is clearly saving a "cannonical" version of the value in "text:p" tag, and Libreoffice is reformating that field at open time, but at save time it keeps the new localizated format there. What I haven't tried is to upload a LibreOffice modified document back to Google Sheets... What should I expect there?