[FrameworkBundle][Translator] Validate locales.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
I know we did this sometimes. But it makes interfaces useless. Image you get an TranslatorInterface service injected. Now you call `getLocale`. According to the interface it does not throw an exception. But you would need to check whether the translator is instanceof FrameworkBundle Translator and maybe catch an exception...