Ask HN: What language should I pursue during summer?

Currently, I am a freshman in High School and I do frontend development and user interface design (HTML, CSS, PS)

I am looking to learn about backend programming over the summer. I currently have a working knowledge of Java (1yr), and basic knowledge of Javascript.

I'm debating over learning Ruby, Python, Javascript (Node.js), or PHP.

My goal is the ability to build web applications, such as a content management system, todo list, etc.

Can I have some suggestions on what to learn? I'll have about 3 months to gain knowledge on the language.

Thanks :)