What programming language is most relevant to IOT

Answer 1

Check out Raspberry Pi and Arduino. Pi uses Python and Arduino uses C/C++.

Answer 2

These days? (Nearly) Any kind of language. You can even program your own IoT devices with scripting languages like JavaScript or even graphical languages like Scratch. Just have a look at Espruino:
https://www.espruino.com/

But "traditionally" it is C/C++ or Java.

However, JavaScript offers you the benefit to prototype faster than traditional languages like Java or C/C++

Good luck! :)