What to translate
I urge folk to only translate Wikipedia, or similar. These 'luxury' or 'car' page translations are not exactly useful to humanity (plus they are rather boring) and I suspect someone somewhere might be getting a fee for for these 'exclusive' pages. If Wikipedia pages aren't offered on clicking 'translations' just go to a follower or 'stream' and you'll see refs to Wiki (or similar) pages that you can click onto.
Useful is a matter of perspective. I don't enjoy car, entertainment and luxury topics, but others do.
If the topic entertains or interests someone else, then why judge? As for the 'fee', at the moment these topics are freely sourced from the internet, but in time there will be paid translations.
It is DuoLingo's business model, you learn a language for free, others get cheap translations.
DuoLingo would cost a fair bit to run (a lot of servers & then staff salaries), and without a plan like that, they won't be able to keep offering a free (and ad free) service like this.
I'm not saying translate what you don't like, but there is no reason to start a crusade against it either ;)
I agree, the fact that there's often nothing to translate on the front page but Fashion and Luxury goods is something that annoys me a lot. Personally I prefer to translate news and science-related articles, like from xatakaciencia, but they're quite rare around here. I don't see many Spanish Wikipedia translations floating around(Though there seems to be quite an abundance on the French side), most of the ones I've seen are about historical people, which are not particularly interesting to me.