Hierarchical layout demands choices be made, but there are advantages in grouping source packages by language: 1) can naturally reflect object packaging models of target language (eg python, jvm). 2) this can encourage reuse of packages across projects.

I've seen this and it works rather well in that one can have per-language bottom level build setup (with autoconf for C, setuptools for Python, ...).

I agree with the GP that it seems weird, and with you that it has its benefits. Of course, other approaches have their own benefits.

Personally, I feel like the top-level directory ordering for a monorepo is somewhat arbitrary, in that you can argue for anything, but it probably doesn’t matter; especially if you have a decent build tool.