For an Accept-Language header like: Accept-Language: en-US;q=0,en;q=0.8,de-AT,de;q=0.9, HttpHeaders.getAcceptableLanguages() returns the list [en_US, en, de_AT, de].
The specification of getAcceptableLanguages says that the returned list is "sorted according to their q-value, with highest preference first", so the expected list is [de_AT, de, en, en_US].