Write an article based on this "Press your phone's Home button. Open WhatsApp. Tap Chats. Tap a conversation. Tap the chat field. Select the Devanagari keyboard. Type your message like usual."
Doing so will minimize Settings. It's a green app with a white phone icon. It's at either the bottom of the screen (iPhone) or the top of the screen (Android). If WhatsApp opens to a conversation, tap the "Back" button in the top-left corner of the screen to view the "Chats" page. Doing so will open the conversation. It's at the bottom of the page. Depending on your phone, this process will vary: iPhone - Press and hold the globe icon in the bottom-left corner of the keyboard, then swipe your finger up to select the Devanagari keyboard. Android - Press and hold the space bar or the "Language" button on the left side of the keyboard, then tap the "Hindi" option. Your keyboard and the characters on-screen will display in Hindi. When you're done typing your message, tap the "Send" arrow to the right of the chat field to send the message.