fix: Document LANGUAGE env var

[RubenGarcia]

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• [ X] I have read the README and Contributor Guidelines

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Description

Document the environment variable LANGUAGE which session already honors.
Fixes #699.

I did not add a formal test but the new file is valid Markdown.

[RubenGarcia]

Is there anything else you'd like added to the doc folder?

I still need to find information about how these env vars are used.