It wasn't already written in prolog. It just would've been much easier and more straightforward for that particular application to write the business logic side in prolog. From a UI standpoint, I've never used prolog for a GUI so it would've needed to be embedded in something else (likely C# given the office preference) anyways, so the decision was to put everything in C# and skip my prolog idea. If I still had those notes, I basically translated handwritten prolog-esque code into C# anyways.

EDIT: In very corporate offices like that one, you don't get to use <language of choice> unless it's staying strictly internal (this one had the potential to be released to customers), you have low oversight and can get away with it (not my case), you have a history of choosing wisely (I had little history with them at that point). They prefer to minimize the risk on the maintenance end (NB: doesn't mean they do what's needed to make maintainable code, they do what's needed to make it easy to hire maintainers by sticking to popular languages and frameworks instead).