I worked in investment banking as a quant. Once had the misfortune of working with a regulator-approved custom built market risk system from the 90s. It had code in every language you can imagine, from cryptic Perl, to Java, to C. The thing worked, but development cycles were a year long, primarily to allow for regression testing. It was a monster to debug, as very few people have the brain space to simultaneously comprehend that many moving parts.