Comment by whazor

A language is normally defined by a grammar and can be parsed. The definition can be very formal.

Alternatively a library is a construct in a pre-existing programming language. For example in Python you pip install a package, import some classes and functions.

But where these concepts can overlap is that you could define your domain logic as a language, or by calling functions in a library. In the past, I was a big believer in domain specific languages. However, AI will outperform when generating Python code with using a new library.