Add i18n support for additional languages

DonFlow currently supports Korean and English. Adding more languages would help reach a wider audience.

**Currently supported:** ko, en
**Requested:** ja (Japanese), zh (Chinese), es (Spanish)

**How to contribute:**
1. Copy `src/i18n/en.ts` 
2. Translate the key-value pairs
3. Register in `src/i18n/index.ts`

**Good first issue** — no app logic changes needed, just translation work.