It is important mostly for you than for the company, because the company runs a business and unless you are joining a new startup, you will be working with code that is already in place so it might make you unhappy in the long run or just impact your output as an employee.
But also taking your example, let's say you are super good with C, well then use C and make clear that even when you don't meet all the criteria for the position you are willing to learn and push yourself to catch up.
When I was looking for a job I did a list of the programming languages I was OK with, then I found companies working with that.
It is unrealistic to think you're going to use all of those at the same time, probably they know that at some point you will have to go and fix a bug in an old project written in R even when all the new stuff is in python, companies are quite reluctant to rewrite what they have because time is money and development time is quite expensive, the business side of things is something that you also need to understand because companies value that a lot.
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