Is it safe to replace a switch with a dimmer on halogen spots?
Unfortunately, my lack of Dutch comprehension prevents me from giving a definitive statement on your situation (do you have an English translation for those switches?).  That said, I live in England and all of the halogens (with their transformers) in my flat are on dimmer switches, so it is absolutely possible to have dimmer switches on your lights.

However, you are going to have to make sure that the transformers that are currently installed in your house will work with a dimmer switch (it is possible that yours will not) by checking the manual or looking it up on the internet.

And definitely take care when working with your electrical system.  If you at all feel uncertain or uncomfortable, it may be worth the peace of mind to talk to or hire a professional.