"The Italian tourists are where the Polish ones are."
Translation:Az olasz turisták ott vannak, ahol a lengyelek.
Well, no offence, this sentence just doesn't look any similar to a grammatically valid Hungarian sentence, that's why it's hard to even touch... Where are the clauses? You either have "az olasz turisták" as a clause (it lacks a predicate) or you have "az olasz turisták vannak", an odd statement, with a strange interjected subordinate clause with an implied unknown verb... Neither is understandable really.
I think the misconception is that once you could somehow get rid of "ott", there would be no need for "van" either. Neither the premise, nor the implication is the right. Technically, you can get rid of "ott" but then the sentence will still be about the location of something, you will still need "van" and in fact, "ott" will still be implied, just at the end of the clause, so your focus will be messy.
You may ask what's with "ahol a lengyelek" then. Well, that's an incomplete clause and this has nothing to do with "van". "Az olasz turisták ott főznek, ahol a lengyelek." would be just as valid. It's just there is no need to repeat certain parts of the sentence, the predicate in particular, when they can be deduced from preceding clauses.
In my opinion it should be "... ahol a lengyel". The english "Polish ones" means "the Polish tourists". In Hungarian the ”ones” is omitted. But the adjective should be in the singular form. The plural form as in the translation suggested by Duo, means ”... where the Polish people are”.