Fixed docs navigation item being mistranslated

[bmispelon]

Certainly! Here is how you would write your description to make it 100% mergeable:

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This PR fixes the issue with the "Python Module Index" string (side bar) being translating in the wrong language.
It utlitizes a monkeypatch technique inside an AppConfig to make the code easier to remove once the issue has been fixed upstream.

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[alexgmin]

I tested in my setup and this fixes the issue of the py-modindex in the docs globalcontext.json always being translated to the first ran language.

Ideally this should have a longer explanation on the bug and a link to the issue in the sphinx repo, but that can be done later if needed.

[bmispelon]

I've opened a bug report against Sphinx: sphinx-doc/sphinx#13448

There's a long description of the bug there, but in summary:

1. The custom json encoder in sphinxcontrib.serializinghtml was not updated correctly after the refactor in sphinx-doc/sphinx@363cdc0 (lazily translated strings are no longer a subclass of UserString)
   2) The string "Python Module Index" comes from a class-level variable (sphinx.domains.python.PythonModuleIndex.localname) and is therefore cached the first time that module is imported. If a language is active at the time of import, the translation will stay cached even if a different language is activated.

I've added a link to the bug report in a comment where the workaround app is added to INSTALLED_APPS.