The semantics don't map onto modern languages: it's verbose assembly, with business logic consisting of GOTOs between segments of million line codebases with debug-evolved behavior encompassing a spec (e.g. translate between ancient ad hoc database formats) and then some. Porting COBOL means replicating all that existing customers expect and depend on, including what's unspecified.
I couldn't tell you how hard it is in practice to work past global state heisenbugs - never written it myself, only studied it out of fascination - but it's an entertaining trainwreck whenever a bank announces a COBOL->Java migration, spends millions, fails, and goes back to COBOL, so it's not even clearly possible in general.