The literature difficulty works for Portuguese, Swedish, Russian, French and a bunch of other languages. It checks for these factors:
- grammatical difficulty of literature - the length of words, sentences, sentence structure, paragraph structure (I think) - all computed in terms of acknowledged scores which exist;
- vocabulary difficulty - frequency vocabulary assessment in terms of different scores and factors, together with tables and data.
Here is a sample of the kind of analysis Interlinear books provides about a book: