[FrameworkBundle][Translator] Validate locales.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
Keep in mind that we are talking about an edge case here where the locale is not valid anyway.

And anyway, we do not document all possible exceptions in interfaces (at least, I would be very surprised if we did it everywhere.) So, that's a non-issue for me.