The correct translation should be "Grandpa is admitted to a/the hospital". "Grandfather is in hospital" on the other hand would translate to "Kakek ada di rumah sakit".
In the US we would never say 'in hospital', that is a Brit locution.
Grandpa is in hospital. Simpler and more commonly used without an article. Reported.
Should also accept "Grandfather went into hospital" or "Grandfather goes into hospital."
In NZ English we do not use "the" here.
If he's already there, we would say "He is in hospital."