We (~150 eng) build microservices in a polyglot environment (mostly Python, JS, and Go), all in a monorepo! We also build + deploy in containers with Jenkins, etc.

The structure looks something like this:

    |-- third_party (third party libs)
    `-- python
        |-- libs (internally-written, common dependencies)
        `-- application_name
            |-- client (react app, connects to flask)
            |-- server (flask app, connects to services)
            `-- services (microservices)

We use a dependency-management + build tool (https://www.pantsbuild.org/index.html, we started before Bazel was public) to manage dependencies. Without pants, our repo would be a mess.

Let me know if you have any questions, I'm happy to answer them! I'm super happy about our setup and eager to share it :)