I'd say Java, because it has a massive footprint amenable for training, and a strong type system (does not have sum types though and those are trendy).
You'd have to steer the LLM to use the style you want, and not massively overarchitect things though, but that's going to be an issue nonetheless.
Java has sum types - they are fairly recent, called sealed records, and can be exhaustively pattern matched on.
(I do agree however, Java is a great target for LLMs)