[Translation][Translator] added getLoaders() method

Except extend the Translator with a useless hack it is not possible to get the list of loaders.

It would be well to allow access to $loaders (protected) property or to a getLoaders()


| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [no]
| License       | MIT
| Doc PR        | [?]