Well you could change the TRUE result to be something other than "". Like maybe 0 (Zero) or " " (blank space). If you use a Zero, you can then conditionally format those cells to make 0's invisible with text color white.
Maybe you should send me the spreadsheet and I can take a look at it tonight.
If so, I'll provide my email address via PM if you don't already have it.
If I download an English Excel program then the conversion to Dutch is automatic. . if I type in English words in the Dutch Spread it is rejected.
You might consider Open Office - a freeware open source product that their "Calc" spreadsheet program is pretty well equipped feature-wise with Excel, but without the hefty tribute to Microsoft.