Change Locale in File > Spreadsheet Settings to ensure correct date format, currency and number format
This change is easy to make. Go to the File menu and select Spreadsheet Settings. Be aware that a document owner and any collaborators with Edit access can make this change to the spreadsheet - and it impacts the way the spreadsheet is seen be everyone.
To change the default locale for all your spreadsheets, you can do so from Document Settings - click the cog in the top right corner and change the Language and Time Zone.