the web gui is constructed using the extjs framework. You can find the details of the webgui in the src/webui directories.
At the moment there is no option to have the webgui work in multiple languages. Most text elements are hardcoded in english in the various files.
If you are willing to work on changing that you are always welcome to do so. Currently tvheadend is using an older version (2.2) of extjs the the latest version available (4.1). It might be that in the lastest version of extjs it is easier to implement a multi language webgui.
Hein