Pick a Hindi voice available on this device. On iPhone you can add "Lekha" under Settings → Accessibility → Spoken Content → Voices → Hindi.

4 digits. Saved on the server, so it works on every device you sign in from.

Progress is saved on this device and synced to the server.

Add this page to your Home Screen (Share → Add to Home Screen) so it opens full-screen like an app, and works without a signal.