i18n: add popular language packs

[AimTune]

Goal

Expand Chativa's translations beyond EN / TR to cover the most-requested languages for chat widgets in enterprise web/SaaS contexts.

What needs translating

Two files per language:

1. `packages/ui/src/i18n/.json` — chat shell, header, input, message list, survey, emoji picker.
2. The `` block in `packages/genui/src/i18n/index.ts` — built-in GenUI components (form, appointment, rating, date picker).

Both must mirror the `en` keys exactly. Add the new resource to `packages/ui/src/i18n/i18n.ts` so `i18next-browser-languagedetector` can pick it up.

How to contribute (per language)

For each language:

1. Copy `packages/ui/src/i18n/en.json` → `packages/ui/src/i18n/.json` and translate every value.
2. Add a sibling block in `packages/genui/src/i18n/index.ts` (copy the `EN` const, translate values, register via `addResources`).
3. Register the resource in `packages/ui/src/i18n/i18n.ts`.
4. Add a sandbox language switcher entry if it isn't already discovered automatically.
5. Native-speaker review preferred — please flag in the PR if you used machine translation only.

Languages to add

Tick when the PR lands. One PR per language is fine; bundling several is also fine.

Latin-script

• Spanish — `es`
• French — `fr`
• German — `de`
• Italian — `it`
• Portuguese (Brazil) — `pt-BR`
• Polish — `pl`
• Dutch — `nl`
• Indonesian — `id`
• Vietnamese — `vi`

Cyrillic

• Russian — `ru`
• Ukrainian — `uk`

CJK

• Japanese — `ja`
• Korean — `ko`
• Chinese Simplified — `zh-CN`
• Chinese Traditional — `zh-TW`

Devanagari

• Hindi — `hi`

RTL (depends on #11)

Translation work for Arabic and Hebrew can start in parallel, but UI must not flip into the wrong direction by accident. Merge order: land the RTL infrastructure issue first, then these.

• Arabic — `ar`
• Hebrew — `he`

Acceptance per language

• All keys present, no untranslated English strings left.
• Plurals (e.g. `survey.starLabel_one` / `_other`) follow the language's plural rules. CLDR plural categories: see https://cldr.unicode.org/.
• Existing tests still pass; if a test references a hardcoded English string, fix the test to assert structurally instead.
• Screenshot in the PR showing the widget in the new language.

Out of scope

• Machine-translation pipeline / translation management platform integration. If this list grows, we can revisit (e.g. Crowdin, Lokalise).