How can we set the default language for request sample ?
Welcome to the community. 
This would be a feature request since as of now setting default language in documentation is not supported.
You can open a feature request here explaining the use-case for it: GitHub - postmanlabs/postman-docs: Documentation for Postman, a collaboration platform for API development. Available for Mac, Windows, and Linux.
Looks like an issue was created on postman-docs https://github.com/postmanlabs/postman-docs/issues/2184 however a maintainer pointed out that was not the right repository to post in. I’ve created another issue in the correct repository if anyone wants to follow along https://github.com/postmanlabs/postman-app-support/issues/8378