Implementing Internationalization (i18n)

# messages_en.properties
order.confirmation.subject=Order {0} confirmed
# messages_de.properties
order.confirmation.subject=Bestellung {0} bestätigt

String subject = messages.getMessage(
    "order.confirmation.subject",
    new Object[]{ order.id() },
    LocaleContextHolder.getLocale());

{count, plural, one {# item} other {# items}}

DateTimeFormatter f = DateTimeFormatter
    .ofLocalizedDateTime(FormatStyle.MEDIUM)
    .withLocale(locale)
    .withZone(user.zoneId());
String s = f.format(Instant.now());

NumberFormat nf = NumberFormat.getCurrencyInstance(locale);
nf.setCurrency(Currency.getInstance("EUR"));
String price = nf.format(amount);  // "1.234,56 €" in de-DE

@Bean
LocaleChangeInterceptor localeChangeInterceptor() {
    var i = new LocaleChangeInterceptor();
    i.setParamName("lang");  // ?lang=de
    return i;
}
@Bean LocaleResolver localeResolver() {
    var r = new CookieLocaleResolver();
    r.setDefaultLocale(Locale.ENGLISH);
    return r;
}