How to create custom (In-app) keyboard?

Please help me to create custom keyboard as shown in screenshot below.

The app that i am currently making need a custom keyboard to input Devanagari text in text box. Is there any extension available or any idea?

your help will be appreciated.

You can create it by yourself without any extension use lables to add your language alphabets and on click add add text in text box that is on that lable