Summary

Commit ef93571 ("Remove translation_*.md") deleted the per-language translation docs (translation_de.md, translation_ja.md, translation_ru.md, translation_uk.md, translation_fr.md, translation_zh-Hans.md), but several references to those files were left behind and now point at 404s. This PR repoints those references at the surviving root translation.md, drops the now-dead "See the X Translation documentation" lines from translation.md, and fixes a couple of small adjacent issues spotted in the same file.

Changes

translation.md

• Drop the four See the [X Translation documentation](translation_X.md) lines (all targets are deleted).
• Fix heading typo ### German transation: → ### German Translation:.
• Fix mislabel ### Traditional Chinese Translation: → ### Simplified Chinese Translation: (the section is for zh-Hans, which is Simplified).
• Fix two stale data-file extensions in the prose:
  • _data/en/stage3.yml → _data/en/stage3.json
  • _data/languages.yml → _data/languages.json
    (Both files are JSON in the repo; the .yml references are leftovers from before the Eleventy migration.)

{de,ja,ru,zh-Hans,uk}/proposals.json
For each locale, repoint two fields at the surviving doc:

• needs_translation_url: …/translation_<lang>.md → …/translation.md
• community_translations: the embedded <a href="…/translation_<lang>.md"> link is repointed the same way.

fr/ and en/ already point at translation.md; no change needed there.

Out of scope (deliberately not in this PR)

• Restoring per-language translation docs as separate files. If maintainers want them back, that's a separate effort and the link targets here can be flipped over again at that point.
• Untranslated strings in de/site.json footer.coc, zh-Hans/site.json footer.coc, and fr/proposals.json specification — those are real but unrelated to dead-link cleanup.

Test plan

• grep -r 'translation_.*\.md' translation.md {de,fr,ja,ru,zh-Hans,uk,en}/proposals.json README.md returns no matches after the change.
• npm run build succeeds; _site/{en,de,fr,ja,ru,zh-Hans,uk}/index.html render normally.
• CI (lint + build) green on this PR.