Från dokumentationen:
The data format string consists of two parts, separated by a colon, in the form {A:Bxx}. For example, the formatting string, {0:D2}, would format the cell to display a number with two decimal places. The entire string must be enclosed in curly braces to denote that it is a format string and not a literal string. Any text outside the curly braces is displayed as literal text.
Jag gissar att du ska skriva nåt i stil med:
DataFormatString="DITT ID ÄR {0:N}"

