Users don't care as long as their requirement are satisfied, but which languages 'best' realise these (TBD) requirements?
If Android, then Java else not(Java)
If Windows, then C# else not(C#)
If millennials, then Python
Note: Windows on AWS is double the price of Linux on AWS so if elastic cloud computing is a big use case, it's better to avoid Windows.
So, OS is the driver of the choice? That's not a good business solution.
Maybe, maybe not.
If the "library" is just a loose collection of code like the CD-ROM in the back of "Numerical Methods" then you are right that the OS is irrelevant.
But if the "library" is a complex application for computing/analyzing financial systems with a beautiful UI and extensive integration with the host machine's software & hardware resources, then picking the first OS is crucial.
And if the library is meant to run in the cloud with some web/internet/command-line interface, then maybe the cheapest option (e.g., Linux) is best because it will save high-end users thousands maybe even hundreds-of-thouands of dollars are year.
But you've never said who the customers or use cases are. Are they PhDs in a hedge fund, newbie code monkeys in a bank's IT department, C-suite suits with a dashboard, or iPhone users trading bitcoins? And what are the use cases: R&D, operational analytics, or production code handling all the bank's transactions?
Or is the library going to be jack-of-all-trades-master-of-none?