[FrameworkBundle][Translator] Validate locales.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
the goal is do document it everywhere. and if we don't then it's by accident. but if we start adding random exceptions to "stable" interfaces that are not documented, then it defeats the whole purpose of adding the annotations completely. to me the goal is to provide the API strictness of java with the programming flexibilty of php. and in any strict typed language (maybe even Hack lang) this would not compile.