The language you know the most would be best fit. Maybe Python/Rails/Go could be most optimal fit, if you know very good Java or PHP and don't want to waste time on learning something new - stay with that.
I'm familiar with Java Servlets and Express for the middle-layer. I've yet to set up a database from scratch, but I've worked with MySQL and Oracle SQL databases before. It'd be cool to learn more about setting up and scaling a database (though I doubt I'd need much scaling for my side-project for awhile).