[HttpRequest] fixes Request::getLanguages() bug

This PR adds to suported languages the first segment of all compouds languages codes.
When receiving `Accept-Language: en-us` header, accepted languages will now be `en, en_US`.

This should not be a BC break as most browsers already send the long **and** short versions of language codes... but some dont.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6928