"The Hungarian tourists are young."
Translation:A magyar turisták fiatalok.
When adjectives are modifying a noun like this, they don't take any endings (plural, accusative, or others). Think of "Hungarian tourists" as one grammatical unit, the subject of the sentence, and it only needs the plural ending once. The ending goes on the noun at the end.
"Predicate nominative" sentences like this, in the 3rd person, ("X is Y" or "X are Y"), where the predicate "Y" is a noun or an adjective, don't include the verb van. (This also applies to the formal "you" forms, maga and Ön, which behave grammatically as 3rd person.)
Öreg vagyok (I am young - 1st person, so you need the verb)
Magasak vagytok (You are tall - 2nd person, so you need the verb)
A turisták fiatalok (3rd person, "X are Y" construct, so you omit the verb)
A birka fehér (The lamb is white, 3rd person, no verb)
Ön nagyon rendes ember (You are a very decent person, formal "you", no verb)
A turisták a téren vannak (The tourists are on the square). In order to assert that someone is somwhere, you need the verb - it's not the predicate nominative type of sentence.