I connected to a VPN and turned off location but telegram still show my country in registration form. How this is possible?
your device is still connected to internet , your mobile network provider . so your location is no longer hidden .
there are different methods to get the user's current location in android. If you have turned off your location it checks your last known location from your phone
Try changing your current location in maps by using/downloading FAKE GPS from play store.
Hope this helps.
If you are using a Smartphone with SIM card inserted and unlocked, Telegram can simply query the network operator info (which includes the country) of your SIM card.