"Museums are important cultural buildings."
Translation:A múzeumok fontos kulturális épületek.
Why are "fontos" and "kulturalis" in singular form when they come after a plural noun?
They are not after a plural noun. They are in front of a noun: "épületek". They do not belong to the subject, they are related to (or part of) the predicate.
And adjectives before nouns stay singular.