It strongly depends on what the company does. When I used to work for Heroku, they hired experts on specific languages for two reasons: 1) to maintain legacy

It strongly depends on what the company does. When I used to work for Heroku, they hired experts on specific languages for two reasons: 1) to maintain legacy systems written in esoteric languages (e.g. Erlang) 2) to maintain programming language runtimes for that language. The first reason I was less happy with, but the second makes total sense. If part of the core product depends on a specific language (e.g. running others' code, performance consulting) I think it's actually better that you hire experts. The only other reason that doesn't bother me is early stage companies or companies that operate in very specialized disciplines who know what their tech stack is and want people to be able to contribute as immediately as possible. In that case hiring a language/stack specialist makes sense, e.g. hiring a Unity engineer at a game studio or a Python/Typescript full stack engineer for a startup with four people whose stack is Python and Typescript. On the other hand, if a mature company with a deep hiring pipeline is freaking out about you not knowing their exact stack, they can go fly a kite.