added method for copying language to content mapper

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation
- [ ] ... <add your own tasks>


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | [yes|no => <WHY?>]
| Fixed tickets | part of https://github.com/sulu-cmf/sulu-standard/issues/125
| BC Breaks     | [description]
| Doc           | [url to commit|none]