What programming language to use entirely depends on several factors:

What languages to do you/the team have experience with? What are the features of the web app? Realtime stuff? Single-Page App? Lots of IO?

I've lately preferred working with Node.js, and Ruby before that.

I know we use Python (with Pyramid) for a lot of internal things at work and the main website is made fully with Java.

The bigger websites of the world use many different languages.

Google seems to favour Python, Facebook uses PHP, Twitter used to use Ruby and now they are mostly Java (?).

So what language you use is not that important in my opinion, you just need to find one that fits your needs. I also prefer to use languages that have a good community around it, that way you can ask others :)