API - I18N and locale detection

We need to add proper I18N of the API because it currently doesn't detect or do anything about the client locale.

It shoudn't be terrible hardly to add: just something along the lines of:

@Context HttpHeaders httpHeaders) {
List<Locale> locales = httpHeaders.getAcceptableLanguages();

in most of the calls, and then pass the first locale down to the service components.