First of all, it is completely up to you which programming language you want to learn. If you are more into web development, PHP is almost a must these days. Especially if you decided to go with the WordPress development, PHP is unavoidable. So I should say, you should learn PHP if you want to develop themes for WordPress.
Java and Python is not used for WordPress development and are mostly used to for mobile applications, other web projects, and software development. So repeating my first sentence. You should have a definite idea about the route you are going to take. And then decide which language to learn. If you choose PHP, I can say that it will be easier to get started with and is a popular programming language as of now.
I just started learning PHP. I love it. It is very easy to learn compared to complex languages I took a quick look at. It is also very popular these days especially because of the high-demand of WordPress. So, I would say, you should learn PHP if you are into web development.