Most of the time you know when a specialist is needed. Also, the specialist's skill tends to be part of the job description, along with the stuff that is unnecessarily restrictive.

You might come across a ML job advert that says the person needs to know Matlab. Well, what if you're a Numpy or R ML guy? It's quite clear to me that such a person is just as well qualified.

If you're looking for a compiler guy, you want someone who understands those optimisations on a theoretical level, ie abstracted away from the concrete language. The toolset may be completely different for each language, but people with equivalent fundamental knowledge should not be separated just by what tool they happened to use.