Earlier quoted context omitted.
Yes, I'm aware. But, just like diffizil is a more formal word, so is difficult . Colloquially, people routinely say hard in English, as in hard problem to solve.
Difficult is a common word in English, and I bet most Germans have never heard nor read the word „diffizil“. Translating „difficult“ to „diffizil“ would be a poor translation IMO.
I know the word and I'm only semifluent in German, so it would never occur to me that it was anything seriously obscure.
(Though this is also tangential and has no real bearing on whether or not to translate the book title as difficult or hard. I would have translated it as hard if someone asked me for some reason.)