I do not think that any native speaker in England would say: "Diabetes is a heavy disease".
I agree. They wrote "heavy" because in Romanina "greu" means "heavy". They could have structured the Romanian sentence differently and it would have made more sense. Like in "Diabetul este o boală serioasă.". It would have been better since "serioasă" means "serious"
It can be, but it can sometimes take this form. For example, you can say "punga e grea" (the bag is heavy) but you can't say "punga e grava". It means that it's difficult to survive/ live with it.
It doesn't make sense to translate it word-for-word. Romanians aay "grea", we say "serious".