If a single language is an option you are a small project that is not facing the problems people on large projects are facing. A monorepo will be easy for you without read the article and the lessons learned.
Come back when you have millions of lines of code, written over decades by hundreds (or thousands) of full time developers.
What a weird take, "millions of lines of code, written over decades" applies to quite many C (or C++) codebases where using a high-level language is not a possibility (and companies that do have such codebases are pretty conservative and don't even talk about Rust no matter how great fit it would be).
In every case I've seen the vast majority might be C, but there are other other languages hidden in there that are hard to find. Many companies would use more languages if it wasn't such a pain. Rust for example would be really nice to use in new code, if only they can figure out how to mix it in.
There is a space between the two types of repositories you are describing. One where you just have enough tools/langs that a single-language setup does not cut it for you anymore, but investing all that effort into Bazel does not seem worth it yet. That is the gap that Grog is meant to fill.
Come back when you have millions of lines of code, written over decades by hundreds (or thousands) of full time developers.