docs(i18n): document language packs across user, dev, and marketing docs

README + USER-GUIDE describe the sidebar picker and current coverage
(home + shared chrome, per-tool bodies pending). DEVELOPER gains a
how-to for adding packs and keys with the parity-test guarantee.
TECHNICAL §10b records the in-house-JSON architecture and locks in the
no-gettext decision (also logged in DECISIONS). REQUIREMENTS reflects
the new interface surface and updated test count. COPY.md adds a
"Language claim" slot so landing/email work can pick it up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/USER-GUIDE.md

@@ -80,6 +80,17 @@ If you skip the Pipeline Runner, follow this order:
 
 The Pipeline Runner enforces this automatically.
 
+### 3.4 Language
+
+The sidebar has a **Language / Idioma** picker. Two packs ship today:
+
+- **English** (default)
+- **Español**
+
+Pick a language once — the choice persists for the session and the picker is visible from every page. Switch any time; the page re-renders in place with no data loss.
+
+**Coverage** (v1.6): home page, tool cards, the upload + analysis panel, the findings list, the Review & Normalize gate prompt, the sidebar picker, and the shutdown screen. Per-tool page bodies (advanced-option labels, column-mapper prompts, dedup review labels) are tracked for future packs — they currently render in English in both modes. If a string you'd expect to switch doesn't, that's a missing pack key, not a bug in the picker; email support with a screenshot.
+
 ## 4. Review & Normalize gate
 
 Every uploaded file is scanned before any tool sees it.