The term boiler does not aptly describe what a hot water heating system heat source is as the water should never boil. So a house water heating appliance is called a boiler because many of them were originally designed as steam boilers.
A calorifier is a somewhat new term for a water heating device. The term ‘calorie’ is more universally applied as calling a water heating appliance a ‘British thermal unit per hour’ device would be inappropriate anywhere other than in a device made by a member of the Empire.
It may take years to catch on as the ‘calorifier’ is absorbed into any language, whereas ‘water heater’ sounds strange and requires different terms in each language.