If you see BOXES or BLANK, you PHONE itself does NOT support Devanagari.
You can type Hindi, Marathi, Nepali and other languages which uses Devanagari Script.
Works only if
1. Hindi Font Installed
2. Device Complex Font Rendering.
Latest Indian firmwares supports these.
All Device above 4.0 Ice cream Sand-witch.
Report any issues or requests here or mail to firstname.lastname@example.org
For beta testing: https://play.google.com/apps/testing/com.sriandroid.justhindi