Translation:The artists are not Hungarians, but English.
Note that adjectives can't be pluralized in English. In Hungarian, the words "magyarok" and "angolok" can be viewed either as plural nouns or plural adjectives, with no difference in meaning. If you view them as nouns, though, you would logically translate them with plural nouns in English: Hungarians and Englishmen (or English people). On the other hand, if you see them as adjectives, then the logical translation would be Hungarian and English. The result ("The artists are not Hungarian, but English") is still grammatically correct, just as "The artists are not tall, but short" would be.
While you are correct that we do not pluralise adjectives (and as such, the singular "Hungarian" should be accepted here as well) the word "magyarok" can function as a noun or an adjective, so "Hungarians" is also correct, if taken as a noun. Both should be accepted.
How can you tell?
"English" here is an adjective, I would say (it certainly isn't the noun "English" that means "English language", as in "English is a difficult language"), and adjectives in English don't inflect for number.
So "I am English" and "We are English" both have the same form, even though one is "singular" and one is "plural".