docs(contributing): translations section + community-maintained policy

Documents the EN/DE maintained tier vs FR/ES/IT/PL AI-initial community-
maintained tier. Includes the PR workflow for fixing an existing locale,
adding a new one, a translator style guide, and a forward reference to
the self-hosted Weblate plan tracked for v1.6.

CONTRIBUTING.md

@@ -124,6 +124,45 @@ Contributors: track `develop`. PRs target `develop`.
 4. Ensure all checks pass (`pnpm typecheck && pnpm lint && pnpm test && pnpm format:check && pnpm build`)
 5. Submit a PR against `develop` with a clear description of the changes
 
+## Translations
+
+HealthLog ships with English (`en`) and German (`de`) maintained by the project, plus French (`fr`), Spanish (`es`), Italian (`it`), and Polish (`pl`) that are **AI-initial, community-maintained**. The maintained locales are reviewed line-by-line; the community-maintained ones surface a small banner inviting users to improve them.
+
+### Improving an existing locale
+
+Translation files live at `messages/<locale>.json`. Each key matches the same key in `messages/en.json` (which is the source-of-truth — never delete a key here without coordinating).
+
+To suggest a fix or improvement:
+
+1. Fork the repository.
+2. Create a branch like `i18n-fr/<area>` (e.g. `i18n-fr/coach-prompts`, `i18n-pl/dashboard`).
+3. Edit `messages/<locale>.json`. Keep the JSON valid; the i18n integrity test will flag missing or extra keys against `en.json` before CI accepts the PR.
+4. Open a PR titled `i18n(<locale>): <area>`. Describe what was unclear, awkward, or wrong in the previous wording.
+
+### Adding a new locale
+
+If you want to introduce a locale that isn't shipped (e.g. `nl`, `pt-BR`, `cs`):
+
+1. Open an issue first using the **Translation request** template. We'll confirm the locale is in scope and decide whether it joins the AI-initial-community-maintained tier or the maintained tier.
+2. Copy `messages/en.json` to `messages/<locale>.json`.
+3. Translate. AI assistance is welcome as a starting point but the PR description should note whether it's an AI-initial pass or a fully human-reviewed one — that information drives the banner state.
+4. Add the locale to the supported list in `src/lib/i18n/locales.ts` (or wherever the registry lives — grep `SUPPORTED_LOCALES`).
+5. The Coach system prompts at `src/lib/ai/prompts/coach-prompt.ts` and `src/lib/ai/prompts/insight-generator.ts` and the doctor-report PDF strings need translation too. Include those in the same PR.
+6. Run `pnpm test -- i18n-integrity` locally before submitting.
+
+### Style guide for translators
+
+- Tone: warm but reserved, curious not pushy. Examples in EN: "Your blood pressure has settled into the green zone" — not "Awesome, your BP is amazing!"
+- Numbers and units stay numeric: "120/80 mmHg" stays "120/80 mmHg" in every locale.
+- Brand terms stay untranslated: HealthLog, Coach, Tagesbriefing (the DE Coach term — leave as-is in EN if it appears).
+- Medical terms: prefer the language community's native medical vocabulary (e.g. "Blutdruck" in DE, "tensión arterial" in ES, "ciśnienie krwi" in PL). When in doubt, mirror the style used in the country's clinical guidelines.
+- Avoid US-specific units (lb, oz, °F) — HealthLog is metric-first.
+- No PII in copy. No real names, no real readings. Sample data uses round numbers.
+
+### Long-term
+
+When the community grows beyond ad-hoc PRs, we'll spin up a self-hosted [Weblate](https://weblate.org/) instance. That replaces the JSON-PR workflow with a web UI but keeps every translation under our own roof — no third-party telemetry. Tracking issue: `v1.6 — Weblate self-hosted on edge-01`.
+
 ## Code of Conduct
 
 This project follows the [Contributor Covenant 2.1](CODE_OF_CONDUCT.md). By participating, you agree to uphold its standards. Reports of unacceptable behavior go to `conduct@bombeck.io`.