Load snippet language (Request -> Fallback shadow language of node)

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes

Depends on #599 and https://github.com/sulu-cmf/sulu/pull/627

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | none