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>

README.md

@@ -1,6 +1,6 @@
 # DataTools
 
-Local CSV / Excel cleaning. CLI + browser GUI, no cloud, no install ceremony.
+Local CSV / Excel cleaning. CLI + browser GUI, no cloud, no install ceremony. GUI ships with English and Spanish language packs.
 
 ## Tools
 
@@ -56,6 +56,10 @@ python -m src.cli_analyze    any_file.csv  [--json]    # scan only
 
 Every CLI runs preview-only by default; add `--apply` to write output.
 
+## Language
+
+The GUI sidebar has a language picker. Packs ship for **English** and **Español** (`src/i18n/packs/`); the choice persists for the session. Adding a language: drop a `<code>.json` next to `en.json` mirroring its key tree, then list it in `LANGUAGES`. See [Developer Guide §i18n](docs/DEVELOPER.md#i18n--language-packs).
+
 ## Review & Normalize gate
 
 Every uploaded file passes through a CSV-normalization gate before any tool sees it. The analyzer flags ~15 issue types (whitespace, NBSP / zero-width chars, BOM, encoding, smart punct, dirty headers, null sentinels, mojibake, …) tagged by **confidence** (high / medium / low) and **fix action**. The GUI shows each finding with Auto-fix / Skip / Customize, a live before/after preview, and an encoding-override picker. Tool pages refuse to load until the gate passes.