Duolingo should provide guidance as to which documents to translate and also as to which sentences in a document to attempt based on your progress in the skills.
Can you also do something like this through the Translations link? Every document in the lists would have some indicator as to how suitable it would be based on your skill tree progress. And when you click on a document, every sentence would have an indicator telling you how suitable it would be based on your skill tree progress.
You could for example use a color gradient that goes from green to yellow to red for this purpose.
Each sentence in a document is evaluated separately, since some sentences are harder than others. When you open a document, you're automatically taken to a sentence that a) needs to be translated and b) is judged to be suitable for your skill level.
The old translation section showed you the level of each sentence, but it wasn't very useful for the user in figuring out what sentences to try, so now that information just lives in the backstage machinery (I assume).