That's one phrase I won't be practising in front of my Polish fiancée
Was going to enter evil union lol
Surely my answer in English is also correct " this relationship is bad" is this same as "this is a bad relationship"
Semantically yes, but as Duolingo wants the sentence to be also grammatically close - I'm afraid it's not the same. That's the general rule here.
How then would you say the relationship is bad in Polish?
Ten związek jest zły.
Ten/nasz zwiazek jest zły. I think this is the best anwser
Is związek only for romantic relationship / marriage, or can it be referring to other types of relationships - for example, friendships, work relationships, or a relationship between two organizations?
It can have all (?) those other meanings if the context allows it. Just see "Związek Radziecki" (USSR) ;)
I would however exclude friendships from that list.
could you translate "zle" with "rotten"?
I mean, maybe in some context, but why, especially here?