If you try doing the same in Excel, you will notice that the result is the date 13.01.2001 and the underlying number representation of this date is 36904. RadSpreadsheet, just as Excel does, parses the value 13-1.1 into a date, but doesn't apply the date format and this is why the result is 36904, instead of 13.01.2001. In other words, changing the value to a date is expected behavior when the culture of the machine is set to German.
If you would like to avoid this behavior, both in Excel and RadSpreadsheet, you need to perform two steps:
1. Set the format of the cell to text, either though code or through the UI, just as you did in the code you shared.
2. Enter in edit mode of the cell and only then paste the value. This way you avoid the paste command, which parses the value even before it reaches the cell. Instead of this, you will use the paste functionality of the cell editor, which will not attempt to parse it. When you exit edit mode, the value will be parsed, but the text format which we set in step 1. will prevent it from parsing to a date.
I hope this explanation will be helpful and suitable for your scenario. Please, let me know if you have further questions or concerns.