Radiator problem

hello

i have a problem with by car (ssangyong rodius 2005 117k mile 2.7L diesel)
my car radiator was all swollen up from the top and the bottom it came out of the car from the bottom by 10 cm
also when i open the water tank there is a sound of releasing pressure
after that i replaced the radiator and changed it with a brand new one but still the sound of releasing pressure when opening the water tank
and i really wanted to know what the problem is and how i can fix the car
because I'm going to east Europe (Italy) with my family and tus need it to be safe and secure to make the holiday
i went to 4 mechanics and the one told i need to change the head gasket and the other told me it was something else


Thanks for reading post i hope you can help