Why every programming language's first program is 'Hello World' ?

Answer 1

You can say greetings from that language.
You can see this:
https://www.thesoftwareguild.com/blog/the-history-of-hello-world/#:~:text=Traditionally%2C%20Hello%20World%20programs%20are,World%20both%20easily%20and%20correctly.
https://en.wikipedia.org/wiki/%22Hello,_World!%22_program
https://www.quora.com/Why-is-HELLO-WORLD-always-taught-in-programming

Answer 2

Just an easy sentence...

If you have any problem
Next try

Hello underworld

Answer 3

Because it is a greeting to everyone in the world may he/she knows you or not or may it is easy also and because "Hello"alone is not a complete sentence so Hello world is a complete sentence. This is the answer in my view