Tolerance: Simply Hard
(Or Easily Hard)
It more closely mirrors the German cadence and I've generally translated schwer as hard. I always understood diffizil to be the correct translation of the word difficult.
Granted, I learned most of my German from my mother (plus a little self study and I lived there for nearly four years in my twenties), not in school. There are no doubt substantial gaps in my knowledge
I think that’s misleading or even wrong. Source: native speaker.
First, it’s a very rare word that would definitely stand out, even in an already formal conversation. More importantly, “diffizil” is mostly reserved for problems that require a lot of fine motor skills or finesse. It would be wrong to say that lifting some large boulder is “diffizil”, for example, except as a joke. You might use it for repairing a watch or a complicated negotiation (“eine diffizile Angelegenheit”), however.
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.)