In a way, it is. Whenever you write some module in your project, you're effectively designing a language. Every function you add is a new verb, every type you define is a new noun[0] - and the relationships between them form the grammar. It may sound trivial, but I think it's an important and underappreciated point. Modularizing and abstracting is done by building languages on top of other languages.

We usually make a distinction between what is a "library" or "module" and what is a "language", by... it's hard to capture the distinction exactly, but my guess is that we reserve the term "programming language" for languages that a) define a large, highly composable set of building blocks, and b) have complex translation into a lower-level language, ultimately yielding executable code. Whatever the distinction could be, in reality, it's a spectrum.

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[0] - Well, a piece of a noun phrase. Maybe. I'm not a linguist.