If you want to write for multiple systems and platforms what is the best language to learn?

Answer 1

java

android, windows, linux, mac

Answer 2

Python is a general purpose programming language that you can use in many places. However in most applications one language can't be substituted for other.
Like Android programming requires Java.
iOS requires objective C or swift.
windows requires .net
Web requires JS and many others.
Systems programming requires c,c++