Are you using the Excel Importer App Store module or some other way to import your data? If you are using the Excel Importer module, I recommend that you use the 'Parser' function to use a microflow to change the comma into a decimap before you attempt to parse into a decimal.
You could create a microflow that will replace the commas in your numerical strings like so:
replaceAll($InputString, ',', '.')
before you use the parseDecimal function.
When it comes to importing excels, there are many reasons you could be seeing the character switch. Perhaps the format of the excel column is incorrect. Using a microflow to parse the information may be your best way to maintain control over the incoming data.