Locales directory needs to be inside the package directory to be installed, so changing the script to place translation files inside seems the best choice. I also added Brazilian Portuguese translation to use as tests.

However, I wasn't able to see the translated theme, so I'm placing this as draft and asking for help.

I tried:
1- building the binary translation file (msgfmt path/to/file.po -o path/to/file.mo); and then
2- building the doc:

git clone https://github.com/python/docsbuild-scripts
python ./docsbuild-scripts/build_docs.py --quick --build-root ./build_root --www-root ./www --log-directory ./logs --group "$(id -g)" --skip-cache-invalidation --theme "$(pwd)" --language pt-br --branch 3.13

I even can see the binary translation file installed:

$ find build_root -name messages.mo
build_root/venv-3.13/lib/python3.13/site-packages/python_docs_theme/locales/pt_BR/LC_MESSAGES/messages.mo

📚 Documentation preview 📚: https://python-docs-theme-previews--224.org.readthedocs.build/