Google India team introduce Transliteration, that support typing in Indian languages in Gmail - Hindi, Tamil, Telugu, Kannada, and Malayalam. This feature which available by default for Indian user, will automatically convert the English word to their Indian language equivalent as you typing words in the way they sound in English.

If this feature does not visible for you, please enable this feature from language setting page by clicking on the ‘Enable Transliteration’.  Please note that this is not the display language, as you can still have this feature while your Gmail shown in your desired language.