Acculturating new developers is one of the main tasks of an organization. I don't think it's very difficult to communicate that some company uses language[s] X[, Y and Z] only.
That depends on the culture of each specific organization. Are there top-down engineering decisions? Is there a push for more team autonomy?
My experience is that many organizations have something of a pendulum swinging between those two positions, so the current state of that balance may change over time.
Also: many new developers, when they hear "microservices", will jump straight to "that means I can use any language I want, right?"
