Translation:I am looking for youths and English people.
I think this is related to "English ones" sounding extraordinarily awkward in English and "fiatal" not being just a random adjective but more like a way to call a young person.
And still, I would say, I can imagine this sentence to be about some kind of pets, maybe dogs. They may have a nationality assigned and they can be young.
As in many other cases, the expected solution to this test question also requires our guesswork. Even if the offered solution is structurally acceptable, its semantic implication assuming the lack of overlap between the sets of English and young people, makes the Hungarian sentence sound an awkward attempt to express the meaning of something like "Fiatalokat keresek, akik egyben angolok".
Hence I wrote "I am looking for young and English people", which was obviously unacceptable as the translation of the given sentence, whereas the correct solution "I am looking for young people and for English people" does reflect the odd idea that there are no young people among the English.
I frankly don't think such an implication exists. It's more just, on one hand, I'm looking for English "ones" (maybe people but not even necessarily tbh) - and I don't care about their age; on the other hand I'm looking for young "ones" - and I'm not interested in their nationality. The two requirements may or may not manifest in one person or something.