[Translator] [FrameworkBundle] Translation loader - ignore locale

| Q             | A
| -------------  | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    |
| Deprecations? | no
| Tests pass?   |
| Fixed tickets | #6765 - partially
| License       | MIT
| Doc PR        | 

The TranslationLoader should ignore(or should have the option to ignore) locale when collecting keys from translation files. This would help identify missing translations, unecessary keys, and make the translation keys consistent.

My idea would be to create the message catalogue with null locale `$catalogue = new MessageCatalogue(null);`. This would mark the cataloge as not locale aware, and it would only contain translation keys.

What do you think?