By changing the locale of your spreadsheet. In the spreadsheet use File->Spreadsheet settings and use e.g. as locale Germany (or any country which uses comas as their decimal operator).
Note the default locale of a newly created spreadsheet is based on the language setting for the Google Docs interface. So if you set the language of the interface to English(US) new spreadsheets would get United States as the default set locale.
That's it! You're all set.
Replace comas with dots on Google Docs. Excel