Assuming you’re referring to how the .env file is interpreted by the npm dotenv package:
The file simply serves as a ‘text’ configuration, which is parsed by the module. The parsed configuration is used as a basis for adding environment variables. So the env file itself is not really written in any programming language.
Parsing rules can be found here: https://www.npmjs.com/package/dotenv#rules
Parsing code can be found here: https://github.com/motdotla/dotenv/blob/master/lib/main.js