There are at least 12 lumia models in 20 countries that5 haven't received officially the denim update
As far as I know, all lumia models in India are supposed to receive denim update
However, there is no mention of denim update for lumia 638.
Did you buy your phone from an official dealer? As you said, the phone is meant for China and therefore getting support for it in India would be tricky.
Try changing region to China and time zone to China as well and the check for updates.