[Translation] added LoggableTranslator.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3015, #2435
| License       | MIT
@fabpot the logger service is optional in Symfony (you can disable MonologBundle without breaking anything. As you asked to make the logger mandatory in the LoggableTranslator, this requires checking first that we have a logger.