How did Kannada language add in iphone system language (not keyboard)

Kannada language is one of the possible languages that can be added as preferred language (it is on your iPhone system by default)

If it's active, you must somehow have added it.

To remove an added language:

• Tap on Settings
• tap on General
• Tap on Language and region
• look at Preferred Language order
• If you want to remove a preferred language: tap on Edit (top right of your screen)
• You'll see minus signs at the left side of the languages
• Tap on the one you want to remove

You can't add system languages to an iPhone, the ones actually available for displaying the interface are listed in the tech specs, and Kannada is not yet among them.

https://www.apple.com/iphone-xs/specs/

To ask Apple to add the one you want, go to

http://www.apple.com/feedback