@NonNull
    static String getPreferredLanguage() {
        final String userLanguage = StringUtils.lowerCase(Settings.getApplicationLocale().getLanguage());
        final String defaultLanguage = StringUtils.defaultIfBlank(StringUtils.lowerCase(Locale.getDefault().getLanguage()), "en");
        return userLanguage + (userLanguage.equals(defaultLanguage) ? "" : "|" + defaultLanguage) + ("en".equals(userLanguage) || "en".equals(defaultLanguage) ? "" : "|en");
    }
Return a pipe-separated list of preferred languages. English and the device default language (if different) will
always be in the list. A user-set language will be first (if set).
main/src/main/java/cgeo/geocaching/connector/oc/OkapiClient.java