How do I reset a locked Apple Watch in Airplane Mode?
Looks like there is a way to erase a locked Apple Watch.


Hold down the side button to see the options to power off the watch.
Force touch
Select 'Erase all content and settings'


I discovered you can force touch on the power off screen from this site:
http://www.barryschiffer.com/how-to-recover-an-apple-watch-stuck-in-a-crash-reboot-loop/

Official link - https://support.apple.com/en-in/HT204567

If this fails, service from Apple can restore and reset devices that are non responsive or even need hardware intervention or repair.