What language is Apple File System (APFS) written in?

Objective-C is the system programming language that Apple uses to design the APFS file system as well as implement OS designs. Unfortunately, macOS is a closed source, so it's unlikely to find the detailed source code of APFS.

However, if you are talking about the specification, it's written in English. You can find the reference here: https://developer.apple.com/support/downloads/Apple-File-System-Reference.pdf. The English-written specification brought out by Apple explains how APFS works with partitions, which it refers to as containers. Meanwhile, it specifies the data structures that correlate to tables, metadata, and the data we refer to as files.