[FEATURE] Use user locale to render content form

__Tasks:__

- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/229
| BC Breaks     | none
| Doc           | none
I would have used `$this->assertArrayNotHasKey('title', $smallResponse)` and so on, would probably result in better messages when the tests are failing.