i18n: RTL smoke tests, Portuguese locale and a translation guide

- Add rtl_smoke_test covering inventory, detail (lot sheet) and settings
  under Directionality.rtl, with an Arabic label and vernacular name.
- Ship pt.i18n.json (Portuguese) as proof the locale pipeline scales beyond
  es/en; wire its language tile in Settings.
- docs/i18n.md documents the Weblate-ready flow for adding locales.

docs/i18n.md

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+# Translating Tanemaki
+
+Tanemaki is multilingual **by design** — international, not any-country-first
+(see the golden rules in [`../CLAUDE.md`](../CLAUDE.md)). Translations live as
+one JSON file per locale, a format Weblate (and any translation platform that
+speaks nested JSON) handles natively.
+
+## Where strings live
+
+```
+apps/app_seeds/lib/i18n/
+  en.i18n.json   ← base locale (source of truth for keys)
+  es.i18n.json
+  pt.i18n.json
+  strings*.g.dart  ← generated, do not edit
+```
+
+- Placeholders use braces: `"{count} entries added"`. Keep them verbatim.
+- Keys are code and never change per locale; only values are translated.
+- Missing keys fall back to English at runtime (`fallback_strategy:
+  base_locale` in [`slang.yaml`](../apps/app_seeds/slang.yaml)), so a partial
+  translation is shippable from day one.
+
+## Adding a locale
+
+1. Copy `en.i18n.json` to `<code>.i18n.json` (e.g. `fr.i18n.json`) and
+   translate what you can.
+2. Regenerate the Dart bindings:
+   ```bash
+   cd apps/app_seeds && dart run slang
+   ```
+3. Add a language tile in
+   [`settings_screen.dart`](../apps/app_seeds/lib/ui/settings_screen.dart)
+   (label = the language's own name, e.g. "Français") plus the `langXx` key in
+   every locale file.
+4. Run the tests: `flutter test` (an i18n test guards against hardcoded
+   strings; the RTL smoke test guards right-to-left layouts).
+
+## RTL and scripts
+
+Right-to-left locales (Arabic, Hebrew, Persian) are first-class: layouts use
+directional insets and are exercised by
+[`rtl_smoke_test.dart`](../apps/app_seeds/test/ui/rtl_smoke_test.dart).
+Locale-specific resources ship with their locale, never hardcoded: OCR
+language packs (`assets/tessdata/`), PDF fonts (`assets/fonts/`), and species
+common names (`assets/catalog/species.json`, a locale-keyed map).
+
+## Weblate
+
+The JSON-per-locale layout is Weblate-ready (component per file, base
+`en.i18n.json`). Hosting is not set up yet; when it is, this page will link
+to the project. Until then, translation PRs/patches are welcome directly on
+the files above.