i18n: translate the landing page #7

The launcher is English-only. Let's support a few more languages — start with one PR per language.

Approach

• Extract user-facing strings in site/app.js and index.html into a tiny dictionary.
• Add a language picker in the footer (defaults to the browser language).
• Persist choice in localStorage.

Welcome contributions for: 🇪🇸 Spanish, 🇫🇷 French, 🇩🇪 German, 🇮🇳 Hindi, 🇨🇳 Chinese, 🇯🇵 Japanese, 🇧🇷 Portuguese.