Add the new language files to the install.xml & remove the not longer used lang strings

Hi @nikosdion 

This PR should fix the both things @infograf768 pointed out here: https://github.com/joomla/joomla-cms/pull/5323#issuecomment-65886885

- We don't need to ship unused languages with a new plugin
- We need to add the new language files to the `install.xml`