To build and run a local installation of JPlag, you can use the pom.xml in this directory (aggregator). It builds JPlag and the available frontends.
To generate single modules run mvn clean generate-sources package in the base directory; if you want a single file then run mvn clean generate-sources assembly:assembly inside the jplag directory. You will find the JARs in the respective target directories. If you build a single JAR, it will be generated in jplag/target.
Installing, running and maintaining a local web service is not recommended as the web service uses outdated libraries and (really) needs polishing.
If you want to do it anyway: atujplag is the client, webservice is the - yepp - web service.
We're happy to incorporate all improvements to JPlag into this code base. Feel free to fork the project and send pull requests.
Adding a new language frontend is quite simple. Have a look at one of the jplag.frontend projects. All you need is a parser for the language (e.g., for ANTLR or for JavaCC) and a few lines of code that sends the tokens (that are generated by the parser) to JPlag.