Can one be a co-translator of a book, if he does not know the language that the book is translated into? made me realize I have a very similar question.
I was translating an extremely slang heavy book from A to B. I had a pretty good grasp on a more formal version of A and native in B and at the time at least I was knowledgeable about the slang of B. Now comes the helper person who doesn't speak B but explained slang A. UrbanDirectory in A, so to speak. What's this helper, then? At which percentage of slang does his work turn from 'explain a few phrases' to 'basically he translated the book from slang-A to A for me'? Is that even the relevant question here? (The translation never got published and this was twenty years ago.)