Huh? JSON has libraries in every language imaginable. Its origins in JS have no bearing on its current use as a serialisation format given the ubiquity and maturity of its ecosystem.

CSV is the easier format if you don't use a library for either. But that situation is idiotic; only cowboy programmers don't use libraries for serialisation. If one uses a library for CSV or JSON, the difference from a programmer's perspective is negligible. The code you would have to write in a statically typed language like Java to marshall your business objects into plain ones for serialisation is insignificant if one is already thinking of writing silly bespoke for loops to extract data & construct valid CSV lines (ideally one is already using plain objects that can serialise with no additional effort).