Does dSLR perform better in hot countries or in cold countries...?

Answer 1

You would have to check the operating instructions for specific cameras. In general I would say they perform worse in cold countries where temperatures are below freezing. There are 2 problems with freezing temperatures:
1- batteries significantly perform worse below freezing, so they will run down a lot quicker (this would be a problem for most modern cameras, not only dslrs). To avoid this keep the camera close to your body to keep the termperature above freezing (when you're not taking pictures). Also bring spare batteries that you keep close to your body.
2- when going back inside condensation may ruin the camera (this would apply to all cameras). To avoid this you can put the camera inside a plastic bag before going inside (so the condensation forms on the plastic bag), or you can first move the camera to a colder room and after a while move it to a heated room.

In hot countries you want to avoid leaving the camera in a car, where temperatures can get very high.

Answer 2

You would probably be more worried with cold countries (battery dies faster in cold, lens element gets condensation) and dusty/sandy countries, dust/sand can get into the body unless you're using a prosessional weather sealed camera and don't change lenses in the field.

Answer 3

IT DON'T MAKE A DIFFERENCE JUST AS LONG AS YOU PUT A FILTER ON THE LENS TO PROTECT IT