Why is there (for ott) not needed in the English sentence?
Because English grammar is not like Hungarian grammar :)
In English, where by itself is fine as a relative connector; it doesn't need an additional there.
I'm not sure whether there's a good answer to the question "why" this is so.
mizinamo says it well! Deep logic suggests to me that "there" is, in fact, where "where" is.
Why do we need to use "are" twice?
Because that is how English works. "The Italian tourists are where the Polish ones" does not make sense in English.
Why doesnt olasz get suffix for plural?
It's an adjective modifying turisták. Adjectives don't get pluralized, unless you make them into nouns. E.g., "olasz turisták" ("italian tourists") but "a turisták olaszok" (the tourists are italian).
And "lengyelek" took over for the whole adjective+noun structure. It has the implied "ones", or "tourists", in it. So that's why it is in the plural.
Hm, then that first sentence in this lesson's tips and notes section -
Hungarian differs from English in that adjectives have to be plural when the subject is plural as well.
should have a pretty large asteriks next to it?
Apparently the context was not so clear that that statement pertains to "predicative adjectives", not attributive ones (which, I believe, are also called "adnomials")..
The Tips& notes got modified by now, now it does not say it always have to be plural. Just sometimes :)
That's still not helpful. It should say that adjectives stay in their base form when modifying a noun, but changes when they take the role as a noun.