Added getLocale()

Added getLocale() to UserInterface.

**Note: The extension of getLocale() is used by `Sulu\Bundle\TestBundle\EntityTestUser`**

Merge with https://github.com/sulu-cmf/SuluTestBundle/pull/6

- [ ] finish the code
- [ ] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none 
| Doc           | none