i18n: Review and polish translations for natural phrasing

[daijir]

Description

We are looking for native speakers and community contributors to review and improve our existing translations across all supported languages to ensure warm, natural, and accurate phrasing.

Target Languages & Files

• 🇪🇸 Spanish (src/locales/es.ts)
• 🇵🇹 Portuguese (src/locales/pt.ts)
• 🇰🇷 Korean (src/locales/ko.ts)
• 🇹🇭 Thai (src/locales/th.ts)
• 🇵🇭 Tagalog (src/locales/tl.ts)
• 🇻🇳 Vietnamese (src/locales/vi.ts)
• 🇹🇼 / 🇨🇳 Chinese (src/locales/zho.ts)
• 🇰🇪 Swahili (src/locales/sw.ts)

Guidelines

1. Check src/locales/<lang>.ts against the master English reference (src/locales/en.ts).
2. Ensure placeholders like {nickname}, {streak}, {count}, {days} remain unchanged.
3. Verify key coverage with npm run check:i18n.
4. Submit a Pull Request with the improved wording!