How do you convince management to treat getting up to speed with a programming language (and your group's particular usage of it) as just another cost of onboar

How do you convince management to treat getting up to speed with a programming language (and your group's particular usage of it) as just another cost of onboarding? Or even better, how do you make the case to whoever is in charge of hiring that they shouldn't bias prior experience to the company's language over newcomers too highly when adequate proficiency is only a matter of a few weeks. All else being equal I think most hiring managers would naturally take the dev who already knows the language, but if it doesn't take long to learn the language up to the company's standards then it's not all else equal, all is effectively equal and another metric is needed.