Change language in backend

Ghost User
2018-05-16T19:49:26+02:00

Add the possibility to change the language in the backend.

### Why?
It is not possible to change the language in the backend.

### Current implementation
It is only possible to change the language in the frontend.
If you want to change the language in the backend, you have to first go to the frontend and change the language there and then go back to the backend.

### Desired implementation
A button to switch the language in the backend, similar to the one in the frontend.