This really depends on what your goal is. If you want to target many platforms as you want, I would said go for Java. But it seems you want to use those engines, most of them have their own scripting language and have an API in C++,. So better learn C++ if you want to use them.
Personally, I would say start with C++ because there is no great difference between the two languages from beginner's perspective. Since you are going to learn them from the very beginning anyway. And select one platform first for your initial project before targeting another platform.