Apple Watch 5 cellular feature international's compatible

A U.S. cellular Apple Watch will not work in India. Different radios in the 2 models

Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

Also the warranty is country specific.

In order for he Cellular feature to work you should buy the Watch in the country where you intend to use it. In other words, buy it in India.