Adjectives are not pluralized when they are part of the noun phrase: "brazil autók"/"Brazilian cars"
They are only pluralized when they are linked to the noun with a verb: "az autók németek"/"the cars are German". Yes, here the verb is left out in Hungarian, but I think the difference should be clear.
"Brazilian or German cars"/"brazil vagy német autók" is a single noun phrase (or a contraction of two noun phrases? I'm not a linguist), so there is no pluralization