Feature/api changes

Added EventListener for Language:

- [x] new route for api
- [x] gather feedback for my changes
- [x] submit changes to the documentation

still open: bug in fosrestbundle makes it mandatory to use pluralization in translationsController (which means that renaming this controller is currently not possible)


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | [url to doc|none]