Definitely learn a new language or library with this project in these conditions. Your website seems like it's not under pressure to ship immediately, so you can take the time to learn something new as part of building the website. You can build a couple of toy ones to start, and iterate to a stronger understanding of the tool you're trying to add to your toolset.
Learning a different programming paradigm is a great way to expand your horizons as a programmer, and leave you better prepared to deal with unusual code later in your career. This is one of the ideas behind the book Seven Languages in Seven Weeks, which would be a great way to stretch your brain. Even if you go for something you know better in this case, grab the book and work on it. It can give you insights into programming problems you might not otherwise have.
Of the options you list, I think learning Scala would probably stretch your brain the most, as it is the most different paradigm to Java on that list. Scala has some strong functional programming aspects, which provide a contrast to the OOP paradigm of Java. Scala works on the JVM, so there are probably plenty of Scala development tools that would plug into tools you're familiar with from your Java development. This could get you up and running sooner, and reduce overall frustration.
I'm primarily a Python programmer, so I think this would also be a good choice. Learning more Python could make you more productive at work. Also, Python is a multiparadigm programming language, which would give you an opportunity to learn elements of functional programming and imperative programming paradigms in a language that you already have some familiarity with. in a language There are a number of web application frameworks for Python with years of history behind them. I'm most familiar with CherryPy, but it's old and weird and you might be better off starting with Pyramid, Django or something simple like WebOb.
Most importantly, however you choose to build this project, keep stretching yourself and challenging yourself. A broader understanding of programming and programming languages makes it easier to later pick up new languages and frameworks when you inevitably need to do so. It's a great investment into a vital programming career.