If you have hired the right person they will very quickly "train" themselves and the only cost to you will be a slightly slower start (which will resolve itself

If you have hired the right person they will very quickly "train" themselves and the only cost to you will be a slightly slower start (which will resolve itself in days to a couple of weeks max) at producing real code. Well worth it if the alternative is hiring someone who knows the language/framework you're working in, but pretty much only knows that language/framework.