What they care about is if you have given them a solution that solves their problem(s). They don't care how you got there. They don't care what technology you used. Just as long as you solved the problem(s) they were having, and that you did it in the time/budget allotment for the job.
That said, as the solution provider you should have found out both their current use cases and potential future use cases, in order to select the tools you'll need for the immediate solution, and ideally for any updates or improvements that may be needed later on.
Because they will care, for instance, if the code you wrote was in a language that won't allow them to scale up quickly and easily (assuming you wrote the rest of the system to allow for this of course), or has some other kind of issue that prevents the client from moving forward with a change.
The last thing a client wants to hear is "complete rewrite using different language/framework/system" just to implement an extension or upgrade path (the best you can usually get by with is a refactor of the existing code base - but you better have a damn good reason and plan for that refactor).
I simply asked a simple question out of curiosity, JFC. Does curiosity matter?
For the record, all code is ephemeral so since you're going to end up throwing it away anyway one day, it doesn't fucking matter what language you start with. Pretty much any language can back-end an API until a certain (large) number of users. #ProblemsWe'dLikeToHave
Twitter threw away Ruby and went to Scala; Facebook is slowly replacing PHP with... whatever, pretty much anything is superior to PHP; plenty of other companies have had to do partial or full rewrites of core functionality on a new stack. Paul Graham himself had his Lisp code tossed in a rewrite.
Uber's ripping out Python and going to a bunch of other langs: https://eng.uber.com/tech-stack-part-one/
Uber and Reddit have schemaless db backends for scalability reasons; I doubt they started out that way.
And yet, Discord is still on Elixir. Isn't that interesting. But I think they're putting in pieces of Rust.
So... I just asked a question.