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>

marketing/COPY.md

@@ -26,6 +26,7 @@ short`) rather than editing in place.
 | Privacy claim | Your data never leaves your computer. |
 | Audit claim | Every change logged to a CSV-format audit trail. |
 | Format claim | $ £ € ¥ R$ kr zł and 50+ phone-country codes — handled. |
+| Language claim | GUI available in English and Español. |
 | Support email | support@datatools.app |
 | Distribution URL | https://datatools.gumroad.com/l/datatools |
 
@@ -188,3 +189,4 @@ ships from a known state.
 | Date | Slot | Old → New | Why |
 |------|------|-----------|-----|
 | 2026-05-01 | (initial) | — | First SoT extracted from landing pages 1.0 |
+| 2026-05-13 | Language claim (new) | — → "GUI available in English and Español." | Ships v1.6 i18n: EN + ES packs in GUI sidebar. Expands addressable market without a CLI/copy rebuild. |