That would be part of the interview process - figuring whether their language of choice is replaceable. Most languages are learnable; every project I've joined was built in a language I had 0 experience in, but it's not hard to just hack in fixes or even rebuild modules completely.

Outsourced work tends to be hacky and unmaintainable for the most part. Sometimes it's built cheap, with no architecture whatsoever or designed for a very specific situation. Even worse, it could be overengineered.