Add locale switcher to DatePicker demo

[jaymantri]

Summary

• replace the hardcoded French DatePicker demo with a locale-switcher example
• add a localized demo that supports multiple locales and week start settings
• keep the change scoped to src/app/page.tsx

Testing

• attempted targeted lint/build validation
• validation is currently blocked in this environment because dependencies are not installed and CENTRAL_LICENSE_KEY is not set for npm ci