HTTP Accept-Language header is specified by the client to inform the backend what the preferred language for the response is. In Java, the go-to utility for handling localization is ResourceBundle.

What is missing is a standard way to properly convert the input header to the correct ResourceBundle. Specifically,

ResourceBundle i18n = ResourceBundle.getBundle("bundles/translations", request.getLocale());

is insufficient. HttpServletRequest::getLocale() method returns the top preferred locale but if no such ResourceBundle exists, it will fall back to default locale instead of going down the priority list. For example, this header:

Accept-Language: de-DE;q=1.0,fr-FR;q=0.9,en-GB;q=0.8

when backend is missing de-DE translations will return the system default (e.g. en-GB) instead of fr-FR which is the second by priority.

Clients don't usually request languages unknown to backend but it is possible in theory and languages can be automatically added by the client platform (iOS does this) without the client knowing.

We need to iterate the locale chain and find the highest match that exists as a bundle.

@RequestScoped
public class Localization {

    @Context
    private HttpServletRequest request;

    private ResourceBundle i18n;

    @PostConstruct
    void postConstruct() {
        //List of locales from Accept-Language header
        List<Locale> locales = Collections.list(request.getLocales());

        if (locales.isEmpty()) {
            //Fall back to default locale
            locales.add(request.getLocale());
        }

        for (Locale locale : locales) {
            try {
                i18n = ResourceBundle.getBundle("bundles/translations", locale);
                if (!languageEquals(i18n.getLocale(), locale)) {
                    //Default fallback detected
                    //The resource bundle that was returned has different language than the one requested, continue
                    //Only language tag is checked, no support for detecting different regions in this sample
                    continue;
                }
                break;
            }
            catch (MissingResourceException ignore) {
            }
        }
    }

    private boolean languageEquals(Locale first, Locale second) {
        return getISO2Language(first).equalsIgnoreCase(getISO2Language(second));
    }

    private String languageGetISO2(Locale locale) {
        String[] localeStrings = (locale.getLanguage().split("[-_]+"));
        return localeStrings[0];
    }

    public ResourceBundle i18n() {
        return this.i18n;
    }
}