You ask which of these sentences is correct:
“This study is humbly dedicated to those whose waterloo is English.”
or
“This study is humbly dedicated to those whose English subject is their waterloo.”
I would say neither is syntactically correct, taking into account that the noun “waterloo” is a generic term for “a decisive or final defeat or setback.” One could have many waterloos in life other than English, so it’s not semantically correct to say “whose waterloo is English” or “whose English subject is their waterloo” in that very specific, single sense. To properly convey the idea of English being just one decisive setback to those being addressed, I think you need an active verb like, say, “find” in that statement and reconfigure the sentence as follows:
“This study is humbly dedicated to those who find English their waterloo.”