Update the Accept-Language header values to include all languages

[NausJessy]

Update the generateLanguageHeaderValue logic to use NSLocale's preferredLanguages property instead of NSBundle's preferredLocalizations.

As per the documentation, the preferredLanguages property returns the user's preferred languages, including the app-specific language (as the first item in the list).

English is no longer added by default. Instead, all identifiers returned by preferredLanguages are included, with an associated priority (q) value, which is calculated depending on the position of the identifier in the list.

q = 1.0 - (position in priority list) * (1 / [list size])