How can I re-use a hole in wall for ventilation?
I realize that this is a U.S. reference, but you may be able to find a wall exhaust fan locally similar to this.


You need to find a unit that is intended for exposure to the outdoors.  Also you may need a cover in the colder months if the unit you find does not have a self closing louver. The one depicted does.

If you use 110 voltage and 60cycles, you may be able to order something like this from the U.S. Otherwise, be sure that it matches your local electric supply.