On wikitech:Developer Portal, document the custom translation plugin used in the Dev Portal.

Notes:

In T297168#7555248, @bd808 wrote:

My initial design for this is plugin is that it should do something like:

1. configure theme languages based on po files found on disk
2. create a temp directory outside of docs_dir for generated files
3. add navigation strings to the English translation catalog
4. generate .md files for all (Page, language) pairs where language != English
5. let mkdocs find all the generated pages
6. if md target language is English, add messages to English translation catalog
7. if md target language is not English, translate md using po

These steps will be scattered across multiple mkdocs callbacks so that each happens in a place where other plugins and mkdocs built-ins can find the generated content and config when needed.