More important than the language is the architecture. I'd recommend using Amazon's AWS services. They have a free starter offering that lets you build and tes

More important than the language is the architecture. I'd recommend using Amazon's AWS services. They have a free starter offering that lets you build and test your application on a small server and then scale it up effortlessly to larger and more instances as required, only paying for what you actually use. If you go with AWS then either Java or PHP would make sense, since there are good SDKs for these languages. Go to the AWS articles page and search for "architecture", there are lots of good resources to get you thinking about options.